Solid Oxide Fuel Cell Anode Research Articles

Application of High-Temperature Raman Spectroscopy (RS) for Studies of Electrochemical Processes in Solid Oxide Fuel Cells (SOFCs) and Functional Properties of their Components

Combined technique and setup have been created that combines capabilities of electrochemical methods, as well as high-temperature Raman spectroscopy (RS) at Osipyan Institute of Solid State Physics RAS. In order to study the processes in the electrochemically active zone, a special geometry of samples was developed on basis of optically transparent single crystal membranes of an anionic conductor with a toroidal shape counter-electrode. With the use of this combined technique and special geometry, studies of the kinetics of reduction of nickel in composite SOFC anodes were carried out. The influence of the composition of fuel on Raman spectra obtained from the internal interface in the current load mode was also investigated, and the correlations with the cell voltage were studied. RS is also used to study the structure of single-crystal samples of anionic conductors, including at SOFCs operating temperature. The new combined technique was used to study other components: sealing glasses for SOFCs and optical glasses.

Process simulation development of a clean waste-to-energy conversion power plant: Thermodynamic and environmental assessment

Waste-to-energy (WTE) conversion technologies for generating renewable energy and solving the environmental problems have an important role in the development of sustainable circular economy. This paper presents a novel high-efficiency WTE power plant using refuse-derived fuel (RDF) as feedstock by integrating torrefaction (T) pretreatment with plasma gasifier (PG), solid oxide fuel cell (SOFC), and combined heat and power (CHP) system. The combined impacts of torrefaction conditions (i.e. temperature and residence time) and steam-to-fuel (S/F) ratio on the energy and environmental performances of the proposed T-PG-SOFC-CHP power plant without CO2 capture (System I) is first evaluated. Results show that torrefaction of RDF prior to plasma gasification provides better syngas quality and therefore the system electrical efficiency (SEE) and CHP efficiency (CHPE) of System I can be markedly boosted compared to that of untreated RDF. However, the integration of torrefaction unit shows a negative effect on the energy return on investment (EROI) due to high energy demands for torrefaction and plasma gasification. Overall, the values of CHPE of System I range from 47.25% to 55.39% when the torrefaction temperatures of 200 and 250 °C are adopted. In contrast, the torrefaction of RDF at 300 °C is not a recommended condition for operation in the T-PG-SOFC-CHP power plant because of noticeably negative energy and environmental impacts. Moreover, to prevent the risk of carbon deposition on the SOFC anode, a recirculation ratio (RR) of the anode off-gas of 30% is required. Finally, the introduction of oxy-fuel combustion technology into the T-PG-SOFC-CHP system for CO2 capture (System II) allows to achieve a zero direct CO2 emission WTE power plant. However, this results in an energy penalty of about 5.40–6.77% associated with the CO2 capture and compression process.

Synthesis, characterization, fabrication, and electrochemical performance of transition metal doped LSCTA- as anode candidates for SOFCS

In this work, lanthanum calcium strontium titanates (La0.2Sr0.25Ca0.45TiO3; LSCTA-) are modified with zinc, manganese and chromium (1%; 5%) for their application as solid oxide fuel cell anodes. These compositions are characterized by X-ray diffraction that indicate successful doping of metals onto the parent (LSCTA-) material without a change in the crystallography and leading to a lattice contraction depending on metal element and amount. Fabrication of the electrodes from the LSCTA- on yttria-stabilized zirconia plates is done by air spraying. The electrode performance of the materials is checked under reducing atmosphere (dry; 5% H2/Ar, humidified; 4% H2O/H2) and at different sintering (1200 and 1250°C) and operating (750–650°C) temperatures. The materials sintered at high temperature (1250°C) show comparatively lower polarization resistances (RP) under humidified atmosphere, hence indicate better electrode performance in the presence of steam. Among these doped compositions, polarization resistance is found lower for LSCTA-modified with 1% Zn than all other doped compositions. Overall, the polarization resistance of both LSCTA-with 1% Zn (23 Ωcm2) and 5% Zn (25 Ωcm2) at 750°Cis found lower in comparison to the value reported at 900°C for parent material (45Ωcm2) under humidified atmosphere.

Structural and Catalytic Characterization of La0.6Sr0.4MnO3 Nanofibers for Application in Direct Methane Intermediate Temperature Solid Oxide Fuel Cell Anodes

In the present work, structural and catalytic characterization was performed on La0.6Sr0.4MnO3 (LSM) nanofibers. The LSM nanofibers were obtained using the electrospinning technique. For comparison, LSM powders with identical composition were characterized as well. The LSM powders were prepared through a self-combustion citrate-based procedure. SEM, EDX, XRD, and BET investigations were carried out on both LSM nanofibers and powders, pointing out the different structural features. The LSM nanofibers showed a higher surface area than the LSM powders and a lower presence of strontium oxide on the surface. Results of the H2-Temperature Programmed Reduction (TPR) tests showed evidence of a higher reactivity of the nanofibers compared to the powders. The catalytic characterization was performed utilizing a methane oxidation activity test, revealing a better catalytic performance of the LSM nanofibers: at 800 °C. The methane conversion achieved with the LSM nanofibers was 73%, which compared well with the 50% obtained with powders at 900 °C.

Quantifying Performance Improvements in MIEC-Infiltrated SOFC Anodes Using a Density of Relaxation Times Analysis of EIS Spectra

To improve the intermediate-temperature performance of solid oxide fuel cells (SOFCs) and expand their applications requires mitigating anode activation polarization while maintaining long-term stability during use. The infiltration of nanoscale electrocatalysts into SOFC anodes has been shown to increase the number of reaction sites, thereby decreasing activation polarization. This study explores the impacts of liquid-phase infiltration of Ni and gadolinium-doped ceria (GDC), a Mixed Ionic and Electronic Conductor (MIEC), into both Ni-YSZ and Ni-MIEC anodes, where the YSZ has been doped with titanium dioxide to facilitate electronic conduction. Distribution of relaxation times (DRT) analysis is performed on Electrochemical Impedance Spectroscopy (EIS) spectra to deconvolute the kinetics of various charge transfer processes. Figure 1

Phase Formation during Reduction-Oxidation of Ni-Substituted Sr(Ti,Fe)O3-δ Solid Oxide Fuel Electrodes

High performance solid oxide fuel cell anodes have recently been achieved via the exsolution of bimetallic (Ni,Fe) particles from a parent Sr(Ti,Fe,Ni)O3-δ (STFN) matrix. However, a number of unknowns remain regarding the detailed nature of the exsolution process, the role of initial A-site deficiency, and the effect of the change in perovskite stoichiometry during exsolution. In this study, the dynamics of exsolution are explored via thermogravimetric analysis (TGA), X-ray diffraction (XRD), and transmission electron microscopy (TEM), both during exsolution and redox cycling. The diffraction results show that the (Ni,Fe) phase starts Ni-rich during the initial exsolution, gradually shifting to more Fe-rich with time; this result indicates that Ni diffusion towards the surface is occurring faster than for Fe. The TGA results suggest that holding STFN under too-reducing conditions can lead to continued Fe exsolution instead of reaching a stable equilibrium. TEM images display regions where SrO formed after Fe-Ni exsolution, likely as a consequence of increased A-site excess in the perovskite. In-situ re-oxidation and subsequent re-incorporation of exsolved species into the parent oxide have been suggested as a means to “reset” exsolution electrodes after degradation by, e.g., particle coarsening. The present TGA results taken during redox cycling suggest that a shorter oxidation step results in faster exsolution, a hint that, in this case, the exsolved surface species do not have sufficient time to fully re-incorporate into the oxide. The effects of new phase formation, changes in exsolved particle composition, and redox cycling on electrochemical characteristics will also be reported.Figure 1. Ex-situ X-ray diffraction scans from Sr(Ti,Fe,Ni)O3-δ after reduction in 30% H2 – 3% H2O – 67% Ar, showing peaks from the perovskite matrix along with a peak associated with exsolved (Ni,Fe) metal alloy; the latter shifts with increasing reduction time indicating that the alloy becomes increasingly Fe-rich. Figure 1

Swarm Intelligence-Based Methodology for Scanning Electron Microscope Image Segmentation of Solid Oxide Fuel Cell Anode

Segmentation of images from scanning electron microscope, especially multiphase, poses a drawback in their microstructure quantification process. The labeling process must be automatized due to the time consumption and irreproducibility of the manual labeling procedure. Here we show a swarm intelligence-driven filtration methodology performed on raw solid oxide fuel cell anode’s material images to improve the segmentation methods’ performance. The methodology focused on two significant parts of the segmentation process, which are filtering and labeling. During the first one, the images underwent filtering by applying a series of filters, whose operation parameters were determined using Particle Swarm Optimization upon a dedicated cost function. Next, Seeded Region Growing, k-Means Clustering, Multithresholding, and Simple Linear Iterative Clustering Superpixel algorithms were utilized to label the filtered images’ regions into consecutive phases in the microstructure. The improvement was presented for three different metrics: the Misclassification Ratio, Structural Similarity Index Measure, and Mean Squared Error. The obtained distribution of metrics’ performances was based on 200 images, with and without filtering. Results indicate an improvement up to 29%, depending on the metric and method used. The presented work contributes to the ongoing efforts to automatize segmentation processes fully for an increasing number of tomographic measurements, particularly in solid oxide fuel cell research.

Enhancing Stability and Catalytic Activity by In Situ Exsolution for High-Performance Direct Hydrocarbon Solid Oxide Fuel Cell Anodes

Enhancing Stability and Catalytic Activity by In Situ Exsolution for High-Performance Direct Hydrocarbon Solid Oxide Fuel Cell Anodes

A thermal grooving study of relative grain boundary energies of nickel in polycrystalline Ni and in a Ni/YSZ anode measured by atomic force microscopy

Grain boundary grooves of nickel were studied in Ni polycrystals and in Ni/YSZ (nickel/yttria-stabilized zirconia) anode microstructures of an solid oxide fuel cell (SOFC) in order to determine the relative grain boundary energies of nickel. Reliable material parameters are necessary for realistic simulations to model the coarsening of nickel grains in SOFC anodes. However, the reported values in literature do not meet the requirements for accuracy and the experimental conditions differ strongly from the conditions within an anode. In this work, the measurement approach for atomic force microscopy was optimized to ensure the required accuracy in measuring grain boundary grooves; the thermal grooving experiments were performed at T=750∘C in dry and humid atmosphere. The resulting distributions of measured dihedral angles and relative grain boundary energies are identical in the polycrystal and the anode microstructure and are independent of annealing time and humidity. For the first time, precise values of the relative grain boundary energies of nickel are determined with high accuracy under operating conditions of an SOFC anode. The mean value of the relative grain boundary energies γGB/γS of nickel is 0.475±0.013 for high-angle grain boundaries, 0.217±0.010 for low-angle grain boundaries, 0.157±0.013 for Σ3 grain boundaries and 0.019±0.002 for twin boundaries.

Modeling of the solid oxide fuel cell anode based on a new analytical model using nonlinear Butler-Volmer expression

Modeling of the solid oxide fuel cell anode based on a new analytical model using nonlinear Butler-Volmer expression

Electrical Conductivity of Electroless Nickel/YSZ deposition for SOFC anode

The paper discusses on the electrical conductivity on nickel-yttria stabilized zirconia (Ni-YSZ) deposition for solid oxide fuel cell (SOFC) anode. The work measures the thickness of Ni-YSZ deposition onto substrate and the electrical conductivity of the anode of SOFC. The thickness of deposition is measured using digital micrometer while the conductivity is obtained by measuring resistance using Fluke multimeter. The factors in this work are particle size and particle condition and using Taguchi method to design the experiment in Minitab 19. It was found out that optimal parameter for high thickness of deposition and high conductivity is nano particle size and uncoated particle condition. The sample using optimal parameter were measured of its conductivity in SOFC environment using furnace up to 700°C. It shows that the optimum condition for SOFC anode is at 700°C to achieved high electrical conductivity. The electrical conductivity performance of Ni/YSZ anode fabricated by electroless nickel deposition was found out comparable to previous study.

Performance Investigation of a Nickel Cermet Anode Modified with Copper, Alkaline Earth Metal Oxide, Boron, and Perovskite for Direct Methane Fuel Cell

The metallic copper, alkaline earth metal oxide, boron, and perovskite were incorporated on the surface of a Ni-cermet anode, and the performance of the modified Solid Oxide Fuel Cell (SOFC) anode was evaluated. The cell performance was analyzed by voltage-current characteristics (V-I curve) and H2-CH4 step reactions (P-t curve) in a potentiostatic mode. Besides, we also determined if a metallic phase or high electronic conductivity of the anode is important for a cell to perform well when H2 is used as a fuel, whereas both conductivity and anti-coking capability are critical while using CH4 as a fuel. The results showed that the anodes containing magnesium oxide (MgO), lanthanum strontium titanate (La0.4Sr0.4TiO3−γ), and boron were relatively resistant to the degradation in the CH4 environment when compared with others. The underlying mechanism varied mainly with electronic and structural promotion by the dopants as well as their material compatibility with the Ni-cermet substrate. These findings were evidenced and supported by surface analysis as well as in-situ infrared and mass spectroscopic studies too.

Direct-Hydrocarbon Proton-Conducting Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are promising and rugged solid-state power sources that can directly and electrochemically convert the chemical energy into electric power. Direct-hydrocarbon SOFCs eliminate the external reformers; thus, the system is significantly simplified and the capital cost is reduced. SOFCs comprise the cathode, electrolyte, and anode, of which the anode is of paramount importance as its catalytic activity and chemical stability are key to direct-hydrocarbon SOFCs. The conventional SOFC anode is composed of a Ni-based metallic phase that conducts electrons, and an oxygen-ion conducting oxide, such as yttria-stabilized zirconia (YSZ), which exhibits an ionic conductivity of 10−3–10−2 S cm−1 at 700 °C. Although YSZ-based SOFCs are being commercialized, YSZ-Ni anodes are still suffering from carbon deposition (coking) and sulfur poisoning, ensuing performance degradation. Furthermore, the high operating temperatures (>700 °C) also pose challenges to the system compatibility, leading to poor long-term durability. To reduce operating temperatures of SOFCs, intermediate-temperature proton-conducting SOFCs (P-SOFCs) are being developed as alternatives, which give rise to superior power densities, coking and sulfur tolerance, and durability. Due to these advances, there are growing efforts to implement proton-conducting oxides to improve durability of direct-hydrocarbon SOFCs. However, so far, there is no review article that focuses on direct-hydrocarbon P-SOFCs. This concise review aims to first introduce the fundamentals of direct-hydrocarbon P-SOFCs and unique surface properties of proton-conducting oxides, then summarize the most up-to-date achievements as well as current challenges of P-SOFCs. Finally, strategies to overcome those challenges are suggested to advance the development of direct-hydrocarbon SOFCs.

Anodic properties of Ni-Fe bimetallic nanofiber for solid oxide fuel cell using LaGaO3 electrolyte

Solid oxide fuel cells (SOFCs) are attracting much attention as alternative energy conversion devices owing to their high energy conversion efficiency and fuel flexibility. Currently, Ni-based cermets or Ni-based bimetal are often being used as anode materials for SOFCs. However, in anode materials, metallic spherical particles generally agglomerate, which affects the electrode reaction under reduction conditions at high temperature. Furthermore, such agglomeration affects both microstructure of the electrode and the cell stability. To overcome this problem, in this study, we designed a bimetallic anode and fabricate it by electrospinning. This Ni-Fe fiber anode exhibits enhanced anodic activity and tolerance to coarsening of metallic particle compared to the Ni-Fe spherical powder. The ohmic and polarization resistance of Ni-Fe fiber anode is lower than Ni-Fe powder anode at all operation temperature. In addition, the single cell using Ni-Fe fiber anode shows higher maximum power densities of 0.40, 0.80, and 1.64 W/cm2 at 973, 1073, and 1173 K, respectively. Such enhanced power generation properties and lower resistance originated from continuous pathways for excellent charge transport pathways generated by electrospinning and the enhanced gas-diffusion properties of the nanofibers. These results demonstrate that the introduction of Ni-Fe fiber anode in SOFCs is an effective approach to enhance their power generation properties and stability.

Durability of Colloidally Stabilized Supported Nickel and Nickel Platinum Nanoparticles during Redox-Cycling

We report on the oxidation and reduction behavior of colloidally stabilized Ni nanoparticles and Pt@NiPt core–shell nanoparticles with a platinum content of 4%. Thin films of both nanoparticle systems were deposited on yttria-stabilized zirconia substrates by spin-coating. Oxidation–reduction cycles were used to remove oxides and organics and obtain metallic particles. The cycling conditions necessary to clean and reduce Pt@NiPt core–shell nanoparticles were milder than for the Ni nanoparticles, which also needed several cycles to burn off residual organics. During cycling, the Ni nanoparticles lost their initially epitaxial relationship with the substrate and adopted a random orientation, while no epitaxial orientation was observable for the core–shell nanoparticles. Reasons for this are discussed together with the influence of platinum on Ni reduction. The Ni and Pt@NiPt nanoparticles sintered during the process but retained crystalline domain diameters close to the original particle diameters. Our results show that colloidally stabilized nanoparticles can be transferred onto a technologically relevant substrate and be reduced to metallic nanoparticles. The fabrication and final structures are discussed as a feasible route to realize solid oxide fuel cell anodes with tailored nickel particle diameter.

Comparative study on solid oxide fuel cell anode microstructure evolution after long-term operation

The evolution of solid oxide fuel cell anode microstructure during operation has important affects on cell performances and degradation. The degradation expectation requires a quantitative evaluation of the microstructure. In this work, anode support unit cells from 4 stacks are sampled and their anode functional layers and supports’ microstructure are evaluated with Focused Ion Beam-Scanning Electron Microscope and optical microscope. The cells are also cut into button cells and perform electrochemical evaluations. These compared microstructures reveal a time-related Ni migration that occurs from the functional layer deep into the support layer and Ostwald ripening. The change of Ni also affects the pore structure in the support layer and influences concentration polarization. The impact of microstructure evolution on electrochemical performance is revealed by Electrochemical Impedence Spectrum tests and Distribution of Relaxation Time analysis.

Operando optical studies of solid oxide fuel cells operating on CO and simulated syngas fuels

Solid oxide fuel cells (SOFCs) are attractive devices for combined power and heat generation because they can operate with a variety of fuels. Syngas (CO + H2) is particularly attractive but many questions remain about its behavior on SOFC anodes including the relative oxidation efficiencies of CO and H2 and tendency to form carbon deposits. In this report, operando spectroelectrochemical measurements, including infrared emission spectroscopy, near infrared thermal imaging, and vibrational Raman scattering, combined with spectrochronopotentiometry, are performed using anode-supported SOFCs operating with simulated syngas at 800 °C. Results quantify the fate of syngas over Ni-YSZ anodes in SOFCs related to their performance. SOFCs operating with simulated syngas mixtures are compared with those for H2 and CO used separately. Thermal imaging shows more SOFC anode heating for H2-rich syngas than for operating on CO or H2 alone. We attribute this result to carbon gasification by water that is produced by H2 oxidation. Raman spectra together with spectrochronopotentiometry experiments illustrate that carbon is not observed on the anode surface at 800 °C – although it is at 750 °C and 700 °C – yet carbon is likely present in the spectroscopically inaccessible, electrochemically active region at the electrolyte-electrode interface.

Three-Dimensional Microscopic Model of Solid Oxide Fuel Cell Anode

In this paper, a three-dimensional microscopic model of the solid oxide fuel cell (SOFC) anode is established based on the SOFC anode microstructure reconstructed by Nano-CT, and the characterization parameters of the electrode are calculated. The electrochemical reaction is defined on the three-phase boundaries of the microstructure, and the polarization characteristic at different temperatures is explored. Results show that the anode polarization loss increase with the increase of temperature. The activation loss and the ohmic loss are of the same magnitude and occupy the main part of the total polarization loss. The pore microstructure directly affects the gas transfer in the anode, and there is a large mass transfer resistance in the small throat, which causes obvious concentration polarization loss and affects the activation loss. Due to the uneven microstructure, the electrochemical reaction rate of the electrodes at the same depth is not consistent. The closer to the electrode-electrolyte interface, the more obvious the unevenness of activation polarization and concentration polarization.

Mo,Cu-doped CeO2 as Anode Material of Solid Oxide Fuel Cells (SOFCs) using Syngas as Fuel

Mo, Cu-doped CeO2 (CMCuO) nanopowders were synthesized by the nitrate-fuel combustion method aiming to improve the electrical and electrochemical properties of its Mo-doped CeO2 (CMO) parent by the addition of copper. An electrical conductivity of ca. 1.22·10−2 S cm−1 was measured in air at 800°C for CMCuO, which is nearly 10 times higher than that reported for CMO. This increase was associated with the inclusion of copper into the crystal lattice of ceria and the presence of Cu and Cu2O as secondary phases in the CMCuO structure, which also could explain the increase in the charge transfer activities of the CMCuO based anode for the hydrogen and carbon monoxide electro-oxidation processes compared to the CMO based anode. A maximum power density of ca. 120 mW cm−2 was measured using a CMCuO based anode in a solid oxide fuel cell (SOFC) with YSZ electrolyte and LSM-YSZ cathode operating at 800°C with humidified syngas as fuel, which is comparable to the power output reported for other SOFCs with anodes containing copper. An increase in the area specific resistance of the SOFC was observed after ca. 10 hours of operation under cycling open circuit voltage and polarization conditions, which was attributed to the anode delamination caused by the reduction of the Cu2O secondary phase contained in its microstructure. Therefore, the addition of a more electroactive phase for hydrogen oxidation is suggested to confer long-term stability to the CMCuO based anode. Keywords: SOFC Anode Material, Ceria, Molybdenum Oxide, Copper Nanoparticles, Syngas Fuel

A highly active CH4 catalyst correlated with solid oxide fuel cell anode performance

Methane catalytic abilities are considered to be an important and direct form of experimental evidence for evaluating the efficacy of MIEC anodes for SOFCs with CH4 as the fuel gas.