Ni-YSZ Anode Research Articles

Nanostructured Lscf-GDC Cathodes Via a Sol–Gel Method for High Performance Solid Oxide Fuel Cells

Cobaltite-based perovskite materials such as La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF), Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) which have rapid surface activation and mixed ionic and electronic ionic conductivity (MIEC) usually mixed with ionic conducting electrolyte material to lower cathodic polarization resistance. Moreover, decreasing the particulate size of the cathode at nanoscale can effectively enhance the ORR activity of the cathode by extending the electrochemically active sites due to its high surface area. However, due the high surface energy, the nanoparticles easily aggregate before mixing into homogeneous heterophase nanoparticles, resulting in significant coarsening between adjacent particles during high-temperature sintering. In this study, GDC and LSCF-GDC composite powders were synthesized via advance sol-gel method. The physicochemical properties such as sintering behavior, phase stability, etc. of the synthesized nanopowders were investigated. The optimal sintering conditions of GDC nanopowders were evaluated based on the relative density and total conductivity of the sintered GDC pellets. Furthermore, the electrochemical performances of the LSCF-GDC nanocomposite cathodes were measured by the electrochemical impedance spectroscopy (EIS). The microstructural characteristics of the LSCF-GDC nanocomposite cathode were quantified via a 3D reconstruction techniques and related to electrochemical performance of the Ni-YSZ anode supported SOFC single cell.

Comparison of Operating Atmosphere Impact on La0.7Sr0.3V3.86-⸹ (LSV) Intermediate-Temperature Solid Oxide Fuel Cell Sulfur Tolerance Explored Through Experimental and Modeling Characterization Over Time

Experts have long projected that in the future solid oxide fuel cells (SOFCs) could provide a viable path for the replacement of existing power sources and the production of clean, secure, and sustainable energy. Today, not only have SOFCs matured as a technology with a proven ability to provide high density, high efficiency power with long term stability at low cost and environmental impact, but with the theoretical capacity to utilize a variety of naturally sourced fuels. This theoretical capacity makes their potential use attractive in many additional applications and positions SOFCs to be the technology which can bridge the gap between current hydrocarbon fueled technologies and future hydrocarbon fueled electric technologies. That said, there remains many technical hurdles associated with the incorporation of naturally sourced fuels, to include thermal stress cracking and catalyst coarsening, delamination, and poisoning. Catalyst poisoning, specifically arising from sulfur impurities in naturally sourced fuels, remains one of the largest SOFC technical challenges, since their presence, even at low concentrations, have now shown permanent performance degradation. 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 impurities presents a concerning technology gap which must be addressed, as the entire fleet operates on JP-8 kerosene-based fuel, which by specification can contain up to 3000 ppm of sulfur, via AR-70 single fuel policy.As SOFC technologies migrate to lower temperature operation to resolve physical degradation issues, the development catalysts materials with greater sulfur tolerance are being explored to resolve issues of chemical degradation related to sulfur poisoning. In this study, a previously reported, high temperature, sulfur tolerant SOFC catalyst, La0.7Sr0.3VO3.86-⸹ (LSV), was further experimentally investigated via Optical Microscopy, X-ray Diffraction, Scanning Electron Microscopy and Energy Dispersive Spectroscopy for its sulfur tolerance at currently targeted SOFC intermediate operating temperatures (400-600°C) in wide range of hydrogen sulfide (30ppm, 300ppm and 10vol%) balance hydrogen gas environments for up to 100 hours. When compared against the industry standard Ni-YSZ anode, LSV was observed to have significantly lower (287x) sulfur adsorption rates, with the lowest rates observed between 600-700°C. This behavior is attributed 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. This study was further supplemented by a theoretical investigation of hydrogen sulfide adsorption on low index LSV surfaces via periodic Density Functional Theory. 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 (0.43 eV max) of undissociated molecular hydrogen sulfide only. The weak chemisorption behavior is attributed to structural surface changes due to oxygen vacancies near strontium defects, wherein the strongest adsorption energies were observed, and in direct contrast to similar calculations of hydrogen sulfide adsorption with low index surfaces of LVO3, wherein no adsorption was observed.Since it is likely SOFCs will operate on hydrocarbon fuels, as well as compressed hydrogen gas in the future, this study is also currently experimentally investigating sulfur accumulation under the same operating conditions above with hydrogen sulfide mixtures in methane gas. Methane is a small hydrocarbon chain, which will help minimize the accumulation of carbon deposition. Initial comparisons of sulfur adsorption between hydrogen and methane gas mixtures, using low hydrogen sulfide concentrations, show they are statistically similar, although methane gas exposure was observed to result in the formation of two activation energies, which was not observed with hydrogen mixture previously. The methane gas was also observed to promote LSV to take the monoclinic/tetragonal structure at lower operating temperatures than hydrogen mixtures. This observation may lead to elevated sulfur adsorption rates at higher hydrogen sulfide concentrations. Finally, it was originally observed during experimental exposure experiments with hydrogen sulfide/hydrogen gas balance mixtures that higher concentrations of lattice oxygen, present at lower temperatures, had a positive correlation with sulfur accumulation in the LSV material. This study is therefore currently theoretically investigating the effect of lattice oxygen content on the LSVs structure via Density Functional Theory, which could lead to further derivation from optimal cubic configuration, which may lead to configurations which increase sulfur adsorption. Figure 1

Reactive Sputtered NiO-YSZ Anode Functional Layer for Thin Film Low-Temperature Solid Oxide Fuel Cell

Solid oxide fuel cell (SOFC) is one of the most promising next-generation electrochemical energy conversion devices because of its wide choice of fuels, cleanliness and, high efficiency of up to 70-85%( with heat utilization). Recently, low-temperature SOFC (< 600 ℃) has been studied to overcome the limitations of conventional SOFC, e.g., thermal stability, cost, and limited application. However, lowering operated temperature is challenging due to slow charge transfer and ion transport rates. Therefore, SOFC design with thin-film stacks, namely thin-film SOFC (TF-SOFC) fabricated on porous anode support, have been usually adopted for sufficient power density even at this temperature regime.A ceramic-metal composite (cermet) of Ni and YSZ (yttria-stabilized zirconia) is the most common anode or anode functional layer (AFL) material of TF-SOFC, because Ni-YSZ cermet has reasonable cost, chemical stability in a reducing atmosphere at high temperature, and similar thermal expansion coefficient to that of YSZ electrolyte. Various thin-film techniques have been reported for AFL fabrication, e.g., pulsed laser deposition (PLD), RF sputtering, etc. Among those, reactive sputtering method, which uses metal target in controlled gas conditions (e.g., oxygen condition for oxide deposition), is a versatile method to deposit various types of thin films with fast deposition speed (up to > 1um/hr), which is beneficial in manufacturability regarding the usual thickness range of AFL (a few microns).In this study, therefore, we studied physical and chemical properties of nanocomposite Ni-YSZ films fabricated by using the reactive sputtering deposition technique, focusing on the effect of sputter parameters. In particular, Oxygen partial pressure significantly affects deposition speed, surface morphology, composition, electrochemical performance, and thermal durability at 450 ℃. It is shown that excessive O2/Ar partial pressure ratio (> 0.2) significantly reduces the deposition speed by approximately one order of magnitude compared to the lower ratio case. In morphological analysis, O2/Ar partial pressure ratio affect nano-structured of anode, after annealing process at 1200℃ with air. Also, the Ni-YSZ anode sputtered at the environment of O2/Ar partial pressure ratio of 0.1 sample shows the lowest activation resistance when applied to LT-SOFC’s anode. The O2/Ar 0.5 sample shows surface Ni agglomeration due to high NiO-to-metallic Ni ratio.

Thermal Investigation of Solid Oxide Fuel Cell Ni-YSZ Anode supported with Cooling System: A FEM Approach

Solid oxide fuel cell (SOFC) is preferred for power generation owing to its high-temperature waste heat recovery and low pollutant emissions. Nevertheless, SOFC operation is highly dependent on an effective cooling system, and the temperature gradient (ΔT) causes the distribution of thermal stress. The trade-off between meeting the SOFC requirement to operate at elevated temperature and minimizing thermal stress by reducing ΔT in the SOFC stack is required. Therefore, a cooling system for the SOFC stack is required to control the temperature homogeneity in the stack. In this study, a 3D finite element method containing heat transfer and energy charge equations was developed and then applied to investigate the effects of the (i) cooling plate and (ii) cooling fin on temperature distribution. The conventional stack orientation (i) parallel flow and (ii) counter flow were considered and compared with the adiabatic model in Ansys thermal analysis. The analysis was carried out under steady-state conditions. Considering that the temperature distribution varies according to the operating temperature and environment, the ambient temperature was set at 800 °C, 750 °C, 700 °C, and 650 °C. The study showed that the steepness of ΔT for a fin with the parallel flow is 1.5% better than for counterflow in the same operating scenario. Notably, the contribution of the cooling plate can minimize ΔT by 10% more than the fin. In addition, cooling measures are required to ensure the long-term stability of the cell during prolonged operation.

Stability of infiltrated cathodes using Pr2NiO4+delta precursor for low-temperature fuel cell applications

The electrochemical performance and stability of infiltrated praseodymium nickelate (PNO) as the cathode for solid oxide fuel cell (SOFC) applications was studied. PNO cathode (Pr2NiO4+δ) was infiltrated into gadolinium-doped ceria (GDC) scaffolds and tested in two different anode-supported cells with tubular geometry: i) tubular cell A with Ni-YSZ support, Ni-YSZ anode functional layer (AFL), and thin (∼7 μm) electrolyte and; ii) microtubular cell B with Ni-YSZ support, without AFL and thick (34 μm) electrolyte. Both cells were stable during 91 h and 260 h of stability testing at 650 °C and experienced a reduction in total polarization resistances. Post-mortem X-ray diffraction (XRD) analysis confirmed PNO's partial transformation into the Pr4Ni3O10 phase with higher electrical conductivity and thermal stability. Improvement in the oxygen reduction reaction was confirmed using DRT analysis. This finding confirms that PNO infiltrated into GDC scaffold can be considered a promising cathode for SOFC applications.

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.

Planar Anode Supported SOFC Fabricated By Sequential Aqueous Tape Casting, Constrained Cosintering and Screen-Printing

Hydrogen technologies and especially fuel cells, are promising alternatives to achieve a carbon-free energy and an ecofriendly transition [1]. The SOFC technology (Solid Oxide Fuel Cell) is an energy conversion system composed of a stack of a porous cathode, a dense electrolyte and a porous anode working at a temperature in the [600°C – 1000°C] range to obtain ionic conductivity above 10-2 S/cm.Nowadays, tape-casting is typically used for mass and low-cost manufacturing of supporting half-cell (8YSZ electrolyte and Ni-YSZ anode) and the cathode layer is commonly deposited by screen-printing [2]. In line with the cost reduction problematic, this presentation will describe the development and optimization of a green sequential tape casting process to obtain the anodic half-cell and screen-printing for the cathode. To target green process, aqueous based slurries are used and only non-toxic organic components are selected. The amount of organic components, ceramic powder and water was optimized to obtain slurries with a shear-thinning behavior while managing the powder dispersion stability. The parameters of the drying heat treatment have been optimized based on Thermal Gravimetric Analysis leading to obtain a multilayer flexible green tape without any bubbles or cracks further annealed to eliminate the organics and co-sinter the stack under a porous YSZ load. The flat anodic half-cell with a thin and dense layer of 8YSZ on a thick and porous layer of NiO-YSZ is completed by screen-printing of La2NiO4+ δ (cathode) based ink followed by a consolidation thermal treatment. The characterization using X-ray diffraction, scanning electron microscopy and operando electrochemical tests of the cell allow validating the efficiency of the sequential tape-casting to fabricate via environmentally friendly process and low cost SOFC.[1] Rissman, J. et al. (2020) ‘Technologies and policies to decarbonize global industry: Review and assessment of mitigation drivers through 2070’, Applied Energy, 266, p. 114848. doi:10.1016/j.apenergy.2020.114848.[2] Udomsilp, D. et al. (2021) ‘Performance Benchmark of Planar Solid Oxide Cells Based on Material Development and Design.’, Energy Technology, 9. doi:10.1002/ente.202001062.

Fracture Simulation of Ni–YSZ Anode Microstructures of Solid Oxide Fuel Cells Using Phase Field Method

The performance degradation of solid oxide fuel cells (SOFC) is directly related to the damage and fracture of electrode microstructures. In this study, the phase field fracture method is used to simulate the fracture of anode microstructures, and the effects of boundary constraints, thermal load, and Ni phase on the fracture of Ni–YSZ anode microstructures are investigated. Results show that tensile stresses occur in the Ni and YSZ phases whether above or below the reference temperature. The cracks propagate along the direction perpendicular to the first principal stress, showing a brittle fracture characteristic. When the microstructure is cooled, all cracks appear in YSZ phase, and almost all cracks initiate at the lowest point of YSZ–pore concave interface. When the microstructure is heated, the tensile first principal stress induces few cracks at local positions but will not make the cracks propagate continuously. The thermal mismatch between Ni and YSZ is not enough to induce cracks, and the fracture of electrode microstructure is more likely to be caused by external tensile load or the thermal mismatch between anode and electrolyte layers. The presence of Ni increases the stiffness of the microstructure, and solid phase’s disconnection reduces the strength of the microstructure.

Highly flexible solid oxide fuel cells using phase-controlled electrolyte support

Flexible solid oxide fuel cells (SOFCs) have attracted increasing attention due to their excellent mechanical stability and lightness. An essential electrolyte material for developing highly flexible SOFCs is 3 mol% yttria-stabilized zirconia (3YSZ), but there remain some difficulties in its application to SOFCs. We report a phase-controlled, bendable, ultra-thin 3YSZ electrolyte with a thickness of ~22 µm, high flexural strength, and improved ohmic resistance that surpasses that of 8 mol% YSZ electrolytes. A flexible cell (total thickness < 60 µm) is fabricated through simple and reproducible methods such as tape casting and screen printing. The highest cell performance is achieved among the reported flexible SOFCs, with the maximum power density of 0.86 W cm−2 at 900 °C using conventional cermet electrodes, Ni-YSZ anode, and LSM-YSZ cathode. Our study provides a well-defined framework for developing flexible SOFCs as next-generation power sources for mobile devices with high volumetric power.

Fine interface contact and high activity of nickel-based anode modified by gadolinium for hydrogen and methane fuel

In this work, gadolinium is used to modify nickel catalyst, which can improve the properties of nickel oxide particle and inhibit its sintering and grain growth. Interface contact between nickel catalyst and YSZ is significantly improved and fine anode microstructure can be obtained when gadolinium is used to modify Ni-YSZ anode. Fine interface contact of GdNi-YSZ anode can accelerate charge transfer process and steam formation process, which leads to high activity for electrochemical oxidation of hydrogen and low impedance resistance. The remarkable characteristic of GdNi-YSZ anode cell is that the cell performance for humidified methane fuel is greatly improved due to the high anode activity for methane reforming and electrochemical oxidation of hydrogen. The maximum power density of GdNi-YSZ anode cell with humidified methane as fuel can reach 1.59 W/cm2 at 800 °C and 0.46 W/cm2 at 650 °C. High performance of GdNi-YSZ anode cell with humidified methane as fuel leads to much H2O produced during the electrochemical oxidation process, which can depress carbon deposition and improve the cell stability for humidified methane fuel.

Achieving Robust Redox Stability of SOFC through Ni-YSZ Anode Layer Thinning and Inert Support Mechanical Compensation

Achieving Robust Redox Stability of SOFC through Ni-YSZ Anode Layer Thinning and Inert Support Mechanical Compensation

Insights on a Ruddlesden-Popper phase as an active layer for a solid oxide fuel cell fed with dry biogas

Solid oxide fuel cell (SOFC) is a mature opportunity for producing power energy in remote areas like islands, where access to the electrical grid is not favoured, and gas distribution is the only viable approach. In this context, generally, biogas represents the most convenient fuel resources in these areas. However, the direct use of biogas in SOFCs is still an issue to be solved due to its negative effect on the conventional Ni-YSZ anode. In this study, to overcome this issue, we suggested using a protective layer coated on the anode of a commercial SOFC. A nickel manganite showing mixed ionic and electronic conductivity tailored specifically for this approach was investigated. The preliminary characterisations showed that the formation of a Ruddlesden-Popper (RP) n = 1 structure supporting fine encapsulated particles based on Ni was formed around 800 °C in consequence of the reducing environment. The electrochemical experiments carried out for 270 h demonstrated for the coated cell significant stability in the presence of dry biogas, albeit an ageing effect was noticed in the electrical percolation of both cell electrodes. The post mortem analyses revealed an attractive redox property for the nickel manganite, which partially returned to the RP n = 2 phase. Moreover, the absence of carbon deposits on the anode suggests possible applications for this approach.

Investigating the Influence of Ni, ZrO2, and Y2O3 from SOFC Anodes on Siloxane Deposition

Siloxanes, as a type of impurity in biogas, can poison the Ni-YSZ anode of SOFCs. However, the influence of individual components of the anode, such as Ni, ZrO2, and Y2O3, on the siloxane deposition process has not been investigated extensively. In this study, Ni, ZrO2, and Y2O3 pellets were exposed to H2 + N2 + H2O + D4 (octamethylcyclotetrasiloxane, 2.5 ppmv) and H2 + N2 + D4 (2.5 ppmv siloxane) gas mixtures at 750 °C to investigate their affinity and tolerance for siloxane degradation. Surface morphology analysis and electrochemical analysis including electrochemical impedance spectroscopy (EIS), related distribution of relaxation times (DRT) analysis and equivalent circuit modeling with complex nonlinear least square (CNLS) fitting were conducted. A microstructure parameter—tortuosity factor to porosity ratio calculated by diffusion polarization resistance was utilized for siloxane deposition evaluation. After comparing pellets surface morphology changes before and after experiments and change following the contamination, Ni is considered as a major factor in siloxane deposition reactions in Ni-YSZ anode.

Electrochemically plated nickel-decorated ceria nanostructures for direct hydrocarbon solid oxide fuel cell electrodes

Nanostructured Ni nanoparticles decorated Sm0.2Ce0.8O2−x (SDC) was uniformly deposited onto a Ni–YSZ anode through two-step electrochemical deposition. The coated anode shows enhanced reactivity and stability relative to the uncoated sample.

Comparison of different infiltration amounts of CeO2 inside Ni-YSZ anodes to improve stability and efficiency of Single-Chamber SOFCs operating in methane

Electrochemically active oxide-based anodes capable of working in Single-Chamber Solid Oxide Fuel Cells (SC-SOFCs) were developed. Their performance is related to the selectivity of the electrodes. Tests are carried out on lab-scale devices with YSZ pellets as solid electrolytes in electrolyte supported cells. Selecting methane as a fuel, a gas mixture in the ratio CH4/O2 = 2 was chosen. The Ni-YSZ (NiO:YSZ=60:40) anode was optimized through CeO2 nanocatalysts infiltration to enhance the anode catalytic activity and make its reduction easier. Several infiltration amounts were compared, from null to 15% of the electrode weight. Both symmetric and complete cells (with LSCF-based cathodes) were tested in H2 and CH4/O2. For increasing amounts of infiltrated CeO2, symmetric cells tests describe an area specific resistance (ASR) reduction from 40 Ω cm2 to 1.7 Ω cm2 in hydrogen and from 11 Ω cm2 to 3.9 Ω cm2 in the methane/oxygen mixture. While complete cells tests displayed an ASR drop from 30 Ω cm2 to 2.9 Ω cm2 in H2, and from 8.7 Ω cm2 to 4.3 Ω cm2 in the methane/oxygen mixture, while OCP and power grew from 478 mV and 3.7 mW cm-2 to 766 mV and 13 mW cm-2.

Production of metal-supported solid oxide fuel cells by co-sintering route

The cost-effective co-sintering route has been considered for the production of Metal-Supported Solid Oxide Fuel Cells in this study. Ferritic stainless steel, composite of Ni-YSZ, and YSZ have been utilized as parent materials for the mechanical support, anode, and electrolyte respectively. The cracking of the multilayers has been avoided by optimizing the de-binding step. The issue of delamination of layers and the interdiffusion of elements have been addressed by the introduction of an ‘intermediate layer’ (composite of LDC and steel) in between the steel support and the Ni-YSZ anode layers. In general, this work presents the optimization of steps and modification of multilayer design and materials towards the successful fabrication of MS-SOFC by co-sintering.

Understanding redox cycling behavior of Ni–YSZ anodes at 500 °C in solid oxide fuel cells by electrochemical impedance analysis

Understanding redox cycling behavior of Ni–YSZ anodes at 500 °C in solid oxide fuel cells by electrochemical impedance analysis

Thermal Residual Stress and Failure Probability of Functionally Grade Ni-YSZ Anode of Solid Oxide Fuel Cells Using a Finite Element Analysis

Due to the differences of thermal expansion coefficient between the components in the solid oxide fuel cells (SOFCs), there is a large residual thermal stress in the fabrication process of the cell. In this study, the functionally graded (FG) Ni-YSZ anode design was introduced between Ni-YSZ support and YSZ electrolyte to mitigate the thermal residual stress in SOFC. In the current FG anode design, composition of Ni and YSZ was varied according the function of φ (Ni) = k ( t / H ) n , where k is a constant which is equal to the volume fraction of Ni in Ni-YSZ anode support layer, t is the distance to the electrolyte, H is the thickness of the FG anode layer and n is the exponential factor. In the numerical model, the FG anode layer was evenly divided into m sublayers. The porosity was set to a constant value of 0.3 and uniformly distributed in the anode. The residual stress of the anode was modeled as the thermal stress when the anode was cooled from the reduction temperature of 1200 oC to room temperature of 20 oC. Residual thermal stress was calculated by finite element method (FEM) using the commercial ANSYS package. Failure probability of functionally graded SOFC was evaluated using the classic Weibull probability model.The effects of sublayer number m = (1, 2, 3, 4), exponential factor n = (0.25, 0.5, ..., 4.0) and gradient layer thickness H = (20, 30, 40 μm) on the stress and failure probability of functionally graded SOFC are discussed. The calculation results show that the FG anode layer can obviously reduce the stress, and effectively decrease the stress concentration between electrode and electrolyte. The failure probability can be reduced by two orders of magnitude at most as compared with the conventional SOFC. Under the same exponential factor n , appropriately increasing the number of sublayers m can reduce the stress of the gradient layer. The sublayers number m = 2 is a suitable value. With the increase of the thickness of the gradient layer, the anode failure probability increases. The research provides a reference for further optimizing the stress in SOFC.

Effect of Fuel Depletion on Ni-YSZ Anode Supported Membrane Fuel Cell Under Different Discharge Modes

Declining performance and service life are important factors restricting SOFC commercial applications. It is necessary to study the influencing factors of material selection, preparation process, operation parameter and failure. During the long-term operation, local gas path failure may lead to insufficient or depletion of fuel gas supply, which may lead to the decline of battery performance or failure directly. During the discharge period, there are two possibilities: gas depletion in a cell unit in stack or in a local region of one cell. In this work, button single cells with Ni-YSZ (anode support)|YSZ (film)|GDC (buffer layer)|LSCF (cathode) structure by mass-producing under industrial conditions were operated with dry hydrogen as fuel. The experiments of fuel gas depletion were carried out by constant current and constant voltage discharge modes to simulate single cell unit failure in series stack and local region failure of one cell. The results show that during constant current discharge, the terminal voltage decreased rapidly and even appeared negative voltage after cutting off the H2 supply. When the H2 was refueled, voltage could rise quickly or even exceed the initial value. However, as the hydrogen was cut off and the constant current was maintained, the performance of the single cell decreased significantly, and the open circuit voltage and maximum power density decreased sharply, which was due to the damage of the cell caused by the anode Ni oxidation-reduction cycles, and cracks appeared on the electrolyte membrane. By contrast, no matter how long the fuel supply was cut off, the single cell performance in constant voltage discharging mode didn’t decline significantly, which indicates that maintaining a reasonable cell terminal voltage can avoid the Ni oxidation in anode caused by fuel depletion. Finally, we conclude that the long time fuel depletion of the cell unit may directly destroy the cell and affect the stack life, while the local fuel depletion of a certain cell will not cause its damage.

Comparative Study of the Performance of Syngas-Fueled Carbon-Air Solid Oxide Fuel Cells Using Anodes Based on M-Doped CeO2 (M: Mo or Mo-Cu)

The use of syngas to feed solid oxide fuel cells (SOFCs) appears as an attractive alternative due to the possibility of being obtained from renewable sources such as biomass or from other solids of carbonaceous composition such as solid wastes through reforming reactions. The use of syngas in SOFCs has been studied in several previous works, directing the efforts to obtain an anode material with the appropriate electrocatalytic activity for the electrooxidation of syngas, able to tolerate carbon deposition and avoid sulfur poisoning [1]. The Ni-YSZ anode has been studied in the electrooxidation of syngas, being one of its advantages the catalysis of the water gas shift (WGS) reaction, generating more H2 with respect to CO and therefore improving the reaction kinetics [2]. One of the disadvantages presented by Ni-YSZ is the decrease of its performance in long term tests, due to the catalytic activity of Ni towards the reverse Boudouard reaction, so that carbon deposits block the triple phase boundaries (TPBs) of the anode [3]. CeO2-based materials are suitable for use as anodes in fuel cells fed with carbonaceous fuels due to their resistance to carbon deposition and sulfur poisoning [4, 5]. Recently it has been reported that the addition of molybdenum to CeO2 (Mo-doped CeO2 or CMO) confers catalytic activity to the carbon gasification reaction and an increase in the electrical conductivity of CeO2 [6]. Furthermore, it has been reported that the addition of copper nanoparticles to Mo-doped CeO2 increases about 10 times its electrical conductivity [7].Two nanomaterials for SOFC anodes were synthesized: Mo-doped CeO2 (CMO) and Cu,Mo-doped CeO2 (CMCuO). Both nanomaterials were synthesized by the combustion method with 10 wt.% total metal loading following the experimental procedures detailed in [6] and [7] correspondingly. The SOFCs were fabricated using a CMO-YSZ or CMCuO-YSZ composite as anode, a dense YSZ electrolyte and a LSM-YSZ cathode. Both SOFCs were fed with air at the cathode and humidified syngas at the anode (syngas + 3 vol.% H2O). The morphology of the SOFCs was studied by SEM, while the crystal structure and chemical stability of the materials were studied by XRD and EDS. The cells were characterized by EIS at OCV condition at different temperatures and syngas concentrations, while the cell performance was studied by linear sweep voltammetry.An ADIS analysis obtained from the EIS data for both fuel cells in Figure 1 clearly shows the presence of two phenomena at high frequencies related to the electrooxidation reactions of H2 and CO, these phenomena are sensitive to temperature changes, the differences in their charge transfer resistance values suggest that both reactions are carried out with different kinetics. The processes shown at low frequencies are related to diffusional limitations of each gas towards triple phase boundaries (TPBs), processes sensitive to changes in syngas concentration. Cell tests, operating at 800°C, showed a maximum electrical power density of 114 mW cm-2 at a current density of 208 mA cm-2 and a cell voltage of 549 mV for SOFC with CMO-YSZ anode and a maximum electrical power density of 120 mW cm-2 at a current density of 200 mA cm-2 and a cell voltage of 600 mV for SOFC with CMCuO-YSZ. The addition of copper to the CMO produces a slight improvement in the electrocatalytic activity of the material and reduced ohmic resistance by almost 50% (see Figure 2), however the ASR of the cell with CMCuO anodes does not show a substantial improvement with respect to the cell with CMO anodes. In addition, the ohmic resistance is the most important component in the ASR of both cells. SEM images of the cells after tests show that the CMCuO anode surface exhibits fractures and delamination of the material, which is not observed on the CMO anode surface. The X-ray diffraction spectra do not show the presence of new phases in any of the two anode materials so the delamination of the CMCuO anode is related to the differences in thermal expansion coefficients (TECs) of the materials whose value must be measured to prove this hypothesis.[1] T.M. Gür (2013) Chem. Rev., 113(8), 6179-6206.[2] O. Costa Nunez et al. (2005) J. Power Sources, 141(2), 241-249.[3] T.M. Gür et al. (2010) J. Power Sources, 195(4), 1085-1090.[4] I. Sreedhar et al. (2019), J. Electroanal. Chem., 848, 113315.[5] N. Mahato et al. (2015) Prog. Mater. Sci., 72, 141-337.[6] I. Díaz‐Aburto et al. (2019) Fuel Cells, 19(2), 147-159.[7] I. Díaz-Aburto et al. (2021), J. Electrochem. Sci. Technol., [epub ahead of print: https://doi.org/10.33961/jecst.2020.01571]. Figure 1