Gadolinia-doped Ceria Research Articles

Boosting the catalytic activity of high-order Ruddlesden–Popper perovskite SrEu2Fe2O7-δ air electrode by A-site La doping for CO2 electrolysis in solid oxide electrolysis cells

Ruddlesden–Popper (R-P) oxides (An+1BnO3n+1) are potential air electrodes for solid oxide electrolysis cells (SOECs) because of their good electrochemical performance. Among them, SrEu2Fe2O7-δ (SEF) with n = 2 has received extensive attention because of its special structure and promising ionic–electronic conductivity. However, the SEF material is prone to degradation at high temperatures (600 °C–800 °C), and its long-term stability is easily affected. In this study, R-P oxide Sr1-xLaxEu2Fe2O7-δ (SLEFx; x = 0, 0.05, 0.1) was successfully synthesized using the sol–gel method. XRD results showed that La-doped SEF displayed good chemical and thermal compatibility with the gadolinia-doped ceria electrolyte. The SLEF0.05 electrode exhibited a polarization resistance of 0.075 Ω cm2 at 800 °C, which was 88.2 % lower than that of SEF. The half-cell based on the SLEF0.05 air electrode exhibited improved performance under the long-term stability test at a current density of − 0.5 A cm−2 for over 110 h at 800 °C, indicating excellent stability. Moreover, an electrolysis current density as high as 0.712 A cm−2 at 1.5 V was obtained for the full cell with the SLEF0.05 air electrode, which was 45.8 % higher than the SEF electrode when the temperature and feed gas were 800 °C and 50 % CO2–50 % CO, respectively. The XPS and cell testing results confirmed that La with high valance and small radius promoted the formation of oxygen vacancies and effectively enhanced the electrochemical performance and structural stability of the SEF material. Therefore, the R-P oxide SLEF0.05 is a promising air electrode material for SOECs.

Experimental Investigation of Naphthalene Induced Degradation of Reversible Solid Oxide Cells Operated on Bio-Syngas

The influence of the operational conditions and the electrode material on the degradation induced by the model tar naphthalene on 10 × 10 cm2 fuel-electrode supported solid oxide cells operated on artificial bio-syngas under internal reforming conditions is experimentally investigated. Two different fuel electrodes are investigated based on yttrium-stabilized zirconia (YSZ) and gadolinium-doped ceria (GDC). A parameter study of the operational temperature and the operation mode (fuel cell or electrolysis mode) is conducted with the YSZ-based fuel electrode to quantify the effects on the cell degradation. The fuel off-gas composition and electrochemical impedance spectra are used to characterize the cell prior to the experiment and monitor its performance during the poisoning process. The influence of the tar on the Ni-YSZ electrode cell is investigated for an operation at 700 °C and 800 °C in fuel cell mode, and an operation at 700 °C in electrolysis mode. The experiment at 700 °C is repeated with the Ni-GDC electrode cell. At the increased operational temperature, the Ni-YSZ cell is more robust but carbon depositions occur. The voltage degradation during electrolysis operation increases significantly compared to fuel cell operation. The Ni-GDC electrode does not show an increased robustness compared to the Ni-YSZ electrode.

Gadolinium Doped Ceria as Nickel–Free Fuel Electrode in High Temperature CO2‐Electrolysis

AbstractIn this work, the performance, electrochemical processes, and stability of GDC (Ce0.8Gd0.2O1.9) fuel electrode containing single cells (GDC/8YSZ/GDC/LSCF) were examined in CO2 electrolysis conditions. The current density and area specific resistance is determined for different operation conditions. The examined cells showed a higher performance compared to state‐of‐the‐art Ni−YSZ fuel electrode (Ni‐YSZ/8YSZ/GDC/LSCF) based single cells with a similar production route. Furthermore, the cells were examined by means of electrochemical impedance spectroscopy (EIS) and the recorded data were evaluated by the distribution of relaxation times (DRT) analysis. An equivalent circuit model (ECM) consisting of four time constants and a Gerischer impedance (LR−RQ1−RQ2−G−RQ4−RQ5) was established to gain further insight into the individual processes in the cells. For instance, the low–frequency process P5 referring to RQ5 is the rate‐determining step in CO2 electrolysis and is assigned to a surface reaction including the charge transfer and possibly the gas diffusion in the GDC fuel electrode. A long–term stability test was performed in CO2 electrolysis conditions at 900 °C with a constant current load of −0.5 A ⋅ cm−2 for up to 1200 h. A degradation rate of 62 mV ⋅ kh−1 was observed. By EIS analysis, a similar contribution of the ohmic and polarization resistances to the degradation was determined. The low–frequency process P5 and thus the surface reaction including the charge transfer in the GDC fuel electrode is contributing most to the increase in polarization resistance. After the long–term stability test, scanning electron microscopy (SEM) analysis was carried out and a coarsening of GDC particles in the fuel electrode was observed.

Unraveling the pivotal role of heterointerfaces on oxide ion transport in solid oxide fuel cells

Overcoming fundamental key issues affecting performance and stability has been recognized as the most important challenge for the commercialization of solid oxide fuel cells (SOFCs). Here, we employed a multipronged approach based on advanced nanoscale analyses coupled with electrochemical evaluation to elucidate the interrelation among ohmic resistance, oxide ion transport, and cation interdiffusion mainly occurring at the interlayer-electrolyte heterointerfaces. The presence of an interdiffusion layer (IDL), formed from the interdiffusion of Ce, Gd, and Zr during high-temperature sintering of conventional screen-printed gadolinia-doped ceria (GDC) interlayer on yttria-stabilized zirconia (YSZ) electrolyte, inadvertently acts as a major barrier to oxide ion transport and results in a significant increase of the ohmic resistance. An effective way to suppress the IDL formation and its oxide ion blocking effect is by alternatively fabricating the GDC interlayer using pulsed laser deposition (PLD), whereby dense layers prepared at a relatively lower deposition temperature without post-growth sintering can be obtained. Consequently, the cell's ohmic resistance is drastically lowered and a significant improvement in cell performance is achieved. These results have major implications for the selection and optimization of cell fabrication processes to mitigate degradation issues associated to heterointerfaces.

Modelling bulk and surface characteristics of cubic CeO2, Gd2O3, and gadolinium-doped ceria using a partial charge framework.

The development and characterization of materials for solid oxide fuel cells (SOFC) is an important step towards sustainable energy technologies. This present study models cubic CeO2, Gd2O3, and gadolinium-doped ceria (GDC) using newly constructed interaction potentials based on a partial atom charge framework. The interaction model was validated by comparing the structural properties with experimental reference data, which were found to be in good agreement. Validation of the potential model was conducted considering the surface stability of CeO2 and Gd2O3. Additionally, the accuracy of the novel potential model was assessed by comparing the oxygen diffusion coefficient in GDCn (n = 4-15) and the associated activation energy. The results demonstrate that the novel potential model is capable of describing the oxygen diffusion in GDC. In addition, this study compares the vibrational properties of the bulk with density functional theory (DFT) calculations, using a harmonic frequency analysis that avoids the need for computationally expensive quantum mechanical molecular dynamics (QM MD) simulations. The potential is compatible with a reactive water model, thus providing a framework for the simulation of solid-liquid interfaces.

In situ synthesis of cobalt-embedded gadolinia-doped ceria nanocatalysts for high-temperature solid oxide cells

Cobalt-embedded gadolinia-doped ceria nanocatalysts are synthesized in situ within a porous fuel electrode, improving the performance and stability of high-temperature solid oxide cells for electricity and hydrogen production.

Gadolinia Doped Ceria with Engineered Grain Boundaries as Potential Electrolyte Material for Thin-Film Solid Oxide Fuel Cells

Thin-film solid oxide fuel cells are receiving increasing attention within the scientific community thanks to their high energy density and high efficiency in different temperature ranges. To be utilized as power supply systems for portable electronic devices, thin-film solid oxide fuel cells must demonstrate high power supply and low-temperature operation capabilities. Both capabilities are highly dependent on the function of the electrolyte layer, commonly comprised of ceramic mixed-ionic-electronic conductors [1]. The state-of-the-art thin-film solid oxide fuel cells utilize yttrium stabilized zirconia as an electrolyte material, despite its relatively low ionic conductivity within the typical working-temperature range of solid oxide fuel cells (up to 1000 °C). Moreover, the ionic conductivity of yttrium stabilized zirconia dramatically reduces with decreased temperature, limiting its performance as an electrolyte material for low-temperature (below 600 °C) solid oxide fuel cells [2].Gadolinia doped ceria is an alternative promising electrolyte material for low temperature solid oxide fuel cells. Its relatively high ionic conductivity below 600 °C makes it attractive as an electrolyte material although its partial electronic conductivity is a significant drawback [3]. This drawback is more pronounced in ultra-thin layers (sub-micron scale) solid oxide fuel cells devices. As a result, gadolinia doped ceria have been utilized only to a limited extent for thin-film solid oxide fuel cells.The issue of partial electronic conductivity might be resolved thanks to a non-conventional deposition technique of polycrystalline gadolinia doped ceria thin films. The proposed sputtering technique includes metallic alloy target and results in a granular morphology, as opposed to the columnar morphology obtained using the conventional sputtering method. As electrical conductivity of polycrystalline thin films is higher along the grain boundaries [4], gadolinia doped ceria layers with random crystallographic orientation are expected to be less electronically conductive. Here, we investigated the integration and the performance of granular gadolinia doped ceria as an electrolyte material for thin-film solid oxide fuel cells. The electronic-conduction properties along the grain boundaries of gadolinia doped ceria were studied, and a thin-film solid oxide fuel cell prototype was fabricated. Such devices with engineered grain boundaries hold the potential for low-temperature operation, high power output, and silicone-compatibility for novel integration opportunities and bulk-fabrication.

Process Intensification and Performance Assessment of Sputtered Gadolinium Doped Ceria (GDC) Barrier Layer in Large-Area Solid Oxide Fuel Cell Short Stack

In recent years, Gadolinium Doped Ceria (GDC) barrier layers have been reported to enhance the performance of Solid Oxide Fuel Cells (SOFCs)by reducing cathode/electrolyte interface diffusion and improving ionic conductivity. Prior investigations have demonstrated that GDC thin film sputtered at room-temperature yields substantial enhancements in Solid Oxide Fuel Cells (SOFCs).This enhancement is evidenced by a noteworthy increase in output current (+78%) and a considerable reduction in ohmic resistance (up to -42%), compared to conventionally screen-printed industrial SOFCs. To probe the intricate interplay between morphology and stoichiometry, which fundamentally determines the Ce3+/Ce4+ ratio governing both ionic and electronic conductivity, this study utilized quantitative X-ray Photoelectron Spectroscopy (XPS) and X-ray Absorption Spectroscopy (XAS) techniques. Results revealed that annealing parameters played a role in the number of available oxygen vacancies in the oxygen-reduction reaction (ORR), inducing different changes in the investigated samples. Non-destructive investigations utilizing Distribution of Relaxation Time (DRT) analysis were conducted on two short stacks with varying thickness, employing different sputtering Modalities. These studies revealed that thickness significantly influences the ultimate performance.

(Invited) Mixed Conduction in Ceramic Electrolytes For Intermediate-Temperature Fuel Cells and Electrolyzers

High-temperature fuel cells and electrolyzers (e.g., T > 700 ˚C) rely on oxide electrolytes such as stabilized cubic zirconia that conduct a single defect, oxygen vacancies. Intermediate-temperature electrochemical cells (e.g., T < 650 ˚C) utilize mixed conducting ceramic electrolytes, that conduct multiple defects. Operating at T < 600 ˚C facilitates lower-cost interconnect materials and balance-of-plant components, but the mixed conductor behavior can reduce fuel cell voltages and lower electrolyzer faradaic efficiencies. Predicting behavior of these mixed conductors, even at open-circuit voltage, requires modeling the coupled transport of the multiple conducting defects in the electrolyte. Detailed models of mixed conductors coupled to porous electrode models can simulate cell performance over a broad range of operating conditions. This presentation highlights models of two types of cells with mixed conducting oxide electrolytes. Firstly, gadolinium-doped ceria (GDC) primarily conducts oxygen vacancies but also some electrons via a reduced-ceria small polaron, but it performs well in intermediate temperature solid-oxide fuel cells [1]. Secondly, yttrium-doped barium zirconates (BZY) primarily conducts protons but also oxygen vacancies and small polarons, which contribute to electronic leakage. Variants of BZY electrolytes perform well in fuel cells and electrolyzers [2-4]. This paper focuses on cell-level models of these mixed-conductors and how to identify favorable regions for high performance in fuel cells and electrolyzers. Zhu, A. Ashar, R.J. Kee, R.J. Braun, G.S. Jackson, “Physics-based model to represent the membrane-electrode assemblies of solid-oxide fuel cells based on gadolinium-doped ceria,” J. Electrochem. Soc., Under revision, 2023.J. Kee, S. Ricote, H. Zhu, R.J. Braun, G. Carins, J.E. Persky, “Perspectives on technical challenges and scaling considerations for tubular protonic-ceramic electrolysis cells and stacks ,” J. Electrochem. Soc. 169:054525 (2022).Zhu, Y. Shin, S. Ricote, R.J. Kee, “Defect incorporation and transport in dense BaZr0.8Y0.2O3-d membranes and their impact on hydrogen separation and compression,” J. Electrochem. Soc., Under revision, 2023.Zhu, S. Ricote, R.J. Kee, “Thermodynamics, transport, and electrochemistry in proton-conducting ceramic electrolysis cells,” in High Temperature Electrolysis, W. Sitte and R. Merkle, Editors, IOP Publishing, 2023.

Impact of Stack Temperature on Solid Oxide Electrolysis System Operation for Lunar Production of Hydrogen and Oxygen Propellants

Solid oxide electrolysis (SOEC) systems have the potential to produce hydrogen and oxygen propellant from lunar ice found in the permanently shadowed cream on the moon. Because SOEC systems electrolyze steam instead of liquid water as in more conventional alkaline and proton-exchange membrane electrolysis systems, they require lower stack voltages for thermal-neutral operation (about 1.29 V/cell vs. 1.48 V/cell for liquid-fed cells) and thus can have lower stack specific energy as defined by stack power requirements per kg of hydrogen produced. However, at a system level, power requirements in a remote lunar setting for vaporizing the steam and flowing it through the stack increase system-level power demands such that the advantages of the SOEC system are reduced due to the balance of plant loads contributing as much as 20 to 25% of the system energy at 800 deg. C. Our team at Mines collaborated with our partners at OxEon Energy to demonstrate an SOEC system with a stack operating temperature of 800 deg. C that demonstrated an overall system-level specific energy just below 50 kWh electric per kg of hydrogen. That study and a supporting model validation study provided a basis for showing the viability for high-temperature SOEC to be scaled-up for lunar production of hydrogen and oxygen propellant for propulsion. The current study builds on that previous work by exploring the impact of SOEC stack temperature on both the stack and balance of plant power requirements. This study explores the trade-offs between reduced balance of plant parasitics and increased stack mass with lower SOEC operating temperatures and explores the advantages of higher conducting mixed-ionic-electronic conducting electrolytes with electron blocking layers to increase current densities at lower temperatures.This study was performed using stack and balance of plant models that have been calibrated in the lab-scale demonstration of an SOEC system with an electrically driven steam generator, a scroll-compressor for driving the steam flow, and cathode and anode exhaust recuperators for heat recovery to the stack inlet steam flow. The system also includes H2 drying from the SOEC cathode exhaust by heat exchange with the incoming water stream. The current study looks updates the previous model by replacing the compressor with a higher pressure steam generator and an expansion valve that allows the operation of the system without the parasitic loads of a compressor. In the current study, three different electrolytes traditional yttrium-stabilized zirconia (YSZ), higher conductivity scandium-stabilized zirconia (ScSZ), and mixed ionic-electronic conducting gadolinium-doped ceria (GDC) with a YSZ blocking layer. The GDC bilayer electrolyte cells have the highest conductivity and allow for reasonable hydrogen production at operating temperatures below 700 deg. C. The results show that the lower operating temperatures enabled by the GDC bilayer electrolyte and by the ScSZ electrolytes not only reduce the system mass with smaller heat exchangers and thermal insulation, but also lower requirements for steam generation by enabling more heat recovery in the steam generator. The impact of stack operating temperatures on the feasibility of scaling up a lunar-deployed SOEC system to produce more than 1 kg/h are presented to further emphasize the feasibility of SOEC systems as part of a lunar propellant production facility.

Advancements in Additive Manufacturing of Protonic Ceramic Fuel Cells

Protonic ceramic fuel cells (PCFCs), also known as proton conducting solid oxide fuel cells, are a promising low-temperature (400-600oC) fuel cell technology. Compared to conventional oxygen-ion conducting ceramic materials, proton conducting ceramic electrolyte materials have lower activation energy (< 50 eV), which can enhance fuel cell efficiency and power output. Additive manufacturing has the potential to revolutionize PCFC manufacturing, as it enables fabrication of both dense and porous structures with desirable mechanical and electrochemical properties. In this study, we employed extrusion-based 3D printing to fabricate PCFCs using BaZr0.3Ce0.5Y0.15O3-δ and BaZr0.4Ce0.4Y0.1Yb0.1O3-δ as electrolytes, NiO as anode, and BaCo0.4Fe0.4Zr0.1Y0.1O3-δ and Ba0.5Sr0.5Co0.8Fe0.2O3-δ as cathode. For comparison, we also printed oxide conducting ceramic fuel cells using gadolinium-doped-ceria (GDC ) as electrolyte. We investigated the printable pastes' rheological properties using dynamic light scattering, viscometer, tensiometry, differential scanning calorimetry, and thermal gravimetric analysis. We also characterized the cells using current-voltage measurements, electrochemical impedance spectroscopy, and various spectroscopic and microscopic techniques (HR-TEM-EELS, SEM-EDX) to understand the underlying mechanisms. Finally, we conducted a systematic study to optimize the sintering temperature for optimal fuel cell performance and investigate degradation mechanisms to improve stability.

A mixed-potential gas sensor with heterojunctions based on Co3O4/ZnO/Y2O3 nanocomposite for low concentration H2S detection

This study aimed to develop a H2S gas sensor based on gadolinia-doped ceria (GDC) solid electrolyte and Co3O4/ZnO/Y2O3 ternary nanocomposite. Heterojunctions were formed inside the Co3O4/ZnO/Y2O3 ternary nanocomposite sensing material due to the difference of work function, which effectively adjusted the oxygen vacancy concentration, improved the electrocatalytic activity of the material, and reduced the working temperature. The response showed a piecewise linear correlation with the concentration of H2S, and its sensitivity for 0.2–2 ppm and 2–10 ppm H2S was −6.4 and −36.35 mV/decade, respectively. Furthermore, the sensor exhibited good stability, repeatability, and selectivity to H2S. This study showed that the GDC-based mixed potential sensor with Co3O4/ZnO/Y2O3-SE could be used as a potential sensor for detecting low concentration of H2S.

Development of BSCMF system of cathode materials and fabrication of symmetrical cell (BSCMF/GDC/BSCMF) for low temperature solid oxide fuel cell applications

ABSTRACT In the present work, cathode materials of Ba0.5Sr0.5(Co1-xMgx)0.2Fe0.8O3 system (x = 0, 0.2, 0.4, 0.6, 0.8, 1) have been produced and the effect of magnesium (Mg) doping level on cathode performance with Gadolinium doped Cerium (GDC) electrolyte was investigated. The polarisation resistance (RP) for cathode symmetric cells was measured from 600°C to 700°C under open circuit state. At 700 °C, the polarisation resistance of BSCF is 13.96 ohm-cm2 and with the addition of Mg it has decreased to 5.17 ohm-cm2. Further increase in the Mg concentration decreased the polarisation resistance up to 0.34 ohm-cm2, which is the minimum value obtained in the composition BSCMF8. It is observed that charge transfer (CT) is more in the case of Mg-rich compositions. With the increase of magnesium concentration in the cathode, RP value was observed as decreased. This is due to the high oxygen permeation flux behaviour of magnesium, which is in the B-site of the cathode. The coefficient of thermal expansion (CTE) of BSCF is measured to be 22.1 × 10−6/K and decreased to 17.45 × 10−6/K with Mg addition in the composition to BSCMF8, due to the constant oxidation state of Mg by which it suppressed the chemical expansion caused by Co.

Microtubular solid oxide fuel cells decorated with gadolinium doped ceria nanoparticles

A simple and feasible method is proposed in this study for the infiltration of gadolinium doped ceria (GDC) into both anode and cathode electrodes of microtubular solid oxide fuel cells (SOFCs) to improve the cell performance. Some crucial infiltration parameters are also systematically optimized according to electrochemical and microstructural investigations along with short-term lifetime tests. The results reveal that the cell performance can be significantly enhanced with nanoparticle GDC decoration. While the reference cell provides only 252.7 mW/cm2 peak power density under hydrogen fuel at 800 °C, the infiltrated cell with the optimized parameters shows 523.0 mW/cm2 maximum power density under the same testing conditions. The noteworthy improvement in the cell performance is found to be as a result of reduced electrode resistances due to increased number of active electrochemical reaction sites resulting from high surface area of nano GDC particles infiltrated as well as GDC being mixed conductor with high ionic conductivity.

Evaluation of Electrolyte Materials of Gd- and Ce-Doped Scandia-Stabilized Zirconia and Yb- and Bi-Doped Gadolinium-Doped Ceria for Highly Durable Solid Oxide Fuel Cells

Evaluation of Electrolyte Materials of Gd- and Ce-Doped Scandia-Stabilized Zirconia and Yb- and Bi-Doped Gadolinium-Doped Ceria for Highly Durable Solid Oxide Fuel Cells

Multi-scale numerical simulation of fracture behavior for the gadolinia-doped ceria (GDC) under mechano-electrochemical coupling fields at high temperature

The fracture toughness of gadolinia-doped ceria (GDC) solid oxide fuel cells (SOFCs) electrolyte with a central crack is significantly reduced under the mechano-electrochemical coupling fields at high temperature. In this work, an atom-to-continuum (AtC) multi-scale method combining the finite element method (FEM) and the molecular dynamics (MD) simulation is developed. Firstly, the AtC multi-scale method is validated by investigating the uniaxial tensile stress–strain curves of GDC with a central crack at different temperatures. Then, based on the theory of fracture mechanics, the macrostructure of GDC is transformed into a microscopic intermediate transition model, and a detailed computational procedure for analyzing the fracture toughness of the macrostructure of the GDC is given by the AtC multi-scale method. Finally, the fracture toughness of GDC macrostructure under the mechano-electrochemical coupling fields is studied by the proposed approach. The simulation results show that the fracture toughness of 10GDC and 20GDC under the mechano-electrochemical coupling fields is clearly reduced compared to the uniaxial tensile loading. Among them, the fracture toughness of 10GDC under the mechano-electrochemical coupling fields is decreased by 12.28% and 30.67% at 800℃ and 900℃, and the fracture toughness of 20GDC under the mechano-electrochemical coupling fields is decreased by 17.25% and 29.52% at 800℃ and 900℃. These findings are critical in predicting the fracture behavior of GDC electrolyte under real working conditions.

Feasibility evaluation of low-temperature deposited thin-film electrolyte with successive post-annealing for solid oxide fuel cells

During the past decade, thin-film-based solid oxide fuel cells (TF-SOFCs) have demonstrated remarkable performance at a low operating temperature of ∼500 °C by reducing ohmic resistance with vacuum-deposited thin electrolytes. However, a high-temperature deposition process at approximately 700 °C is employed in TF-SOFCs, which is only available at a laboratory scale and is not appropriate for commercial deposition equipment. To address this issue, we investigate the feasibility of depositing electrolytes at relatively low temperatures (≤300 °C) accompanied with post-annealing. A TF-SOFC comprising a trilayer thin-film electrolyte, i.e., yttria-stabilized zirconia (YSZ) sandwiched between gadolinia-doped ceria (GDC), fabricated at low-temperature deposition with subsequent post-annealing is selected and tested. Based on a thorough examination using X-ray diffraction and scanning electron microscopy, the appropriate growth conditions for the YSZ and GDC thin-film layers are selected. Through the optimization, we successfully create a low-temperature deposited 750 nm-thick GDC/350 nm-thick YSZ/150 nm-thick GDC (CZC) trilayer electrolyte exhibiting comparable performances with that deposited at 700 °C. High performance of a single cell, a peak power density of 890 mW/cm2 at 500 °C, is achieved. This result suggests the potential of fabricating high-quality TF-SOFCs with commercial equipment.

Effect of metal addition to Ni/CeO2 catalyst on the steam reforming reaction of methane

In this study, the effect of metal addition (V, Co, Cu, Mo, or W) to Ni/CeO2 catalyst on the steam reforming reaction of methane was investigated. The addition of Cu, Mo, or W resulted in decreased catalytic activity. The catalytic activity of the Co added catalyst was not largely affected compared with that of the Ni/CeO2 catalyst. Meanwhile, the addition of V increased the catalytic activity in the steam reforming reaction of methane. Therefore, the V added catalyst was analyzed in detail. Ni–V mixed oxide formation was confirmed in the prepared state. After reduction, it was presumed that the Ni–V mixed oxide was transformed into Ni and the oxide of V. The V oxide was present near the metal-state Ni. The Ni metal phase derived from the Ni–V mixed oxide may contribute to the positive effect of V addition to the Ni/CeO2 catalyst on the steam reforming reaction. Moreover, changing the support to gadolinium-doped ceria (GDC) confirmed the positive effect of V addition. In particular, the Ni–V/GDC catalyst enabled near-equilibrium CH4 conversions even at low temperatures of 600–700 °C.

Thickness effect of thin-film barrier layers for enhanced long-term operation of solid oxide fuel cells

Highly efficient solid oxide cells are one of the most promising technologies for a sustainable future based on renewable hydrogen. The diffusion barrier layer employed between zirconia-based electrolytes and state-of-the-art oxygen electrodes aims to limit the formation of electrically insulating secondary phases that dramatically reduce the cells’ performance. Conventional barrier layers manufactured by screen-printing technology lead to porous microstructures that enable the formation of insulating SrZrO3, partially blocking the active area of the cells. Opposite, homogeneous and dense barrier layers have proven to be the ultimate solution to limit interdiffusion, substantially improving the cells’ performance. Despite the relevance of this solution, the impact of the barrier layer thickness on the final performance of the cells is still unknown. In this work, gadolinia-doped ceria barrier layers with thicknesses between 200 and 800 nm made by pulsed laser deposition were studied in button cells. Excellent electrochemical performance was obtained for all the cells, improving 45% of the power output of the reference counterparts. Moreover, durability tests performed on the cell with the thinnest layer (200 nm) did not show any measurable degradation for 3500 h of continuous operation under high current densities of 0.77 A cm−2 (∼0.87 V) at 750 °C. Post-mortem characterization by synchrotron nano-x-ray fluorescence of a pristine cell and the aged cell allowed us to observe that some spots of SrZrO3 were present at the cathode/electrolyte interface since the cell manufacturing step without increasing during long-term operation. Indeed, the discontinuity of this insulating phase seems not to be critical for cell operation.

Impact of GDC Interlayer Microstructure on Strontium Zirconate Interphase Formation and Cell Performance

In solid oxide cells (SOCs) exhibiting mixed ionic-electronic conductor air electrode (MIEC AE) (e.g. La0.6Sr0.4Co0.2Fe0.8O3–δ LSC(F)), the formation of insulating zirconate interphases at the air electrode/zirconia electrolyte—interface is commonly prevented by an interlayer of doped cerium oxide. This complex interaction has a tremendous influence on cell performance, as the morphology of the interlayer determines the amount and continuity of the zirconate interphases. The performance of fuel electrode supported cells (FESCs) exhibiting gadolinium-doped ceria (GDC) interlayers fabricated from different commercially available powders are compared. All of these layers were fabricated according to the same procedure. The cell performance is analyzed by means of current-voltage (CV) characteristics, electrochemical impedance spectroscopy (EIS) and subsequent impedance analysis by the distribution of relaxation times (DRT). Next to the cell testing at application-oriented conditions, impedance spectra are measured over a wide temperature ranging down to 250 °C to resolve polarization phenomena related to bulk, grain boundary and interfacial effects within and in-between the gadolinium-doped ceria (GDC) and the 8 mol% yttria-stabilized zirconia (8YSZ) electrolyte. Furthermore, symmetrical air electrode (SymAE) cells are analyzed to eliminate impedance contributions from the anode layer (AL) and the substrate. The electrochemical results are correlated to the microstructural features of the GDC/YSZ interface obtained from post-test focused ion beam (FIB)/scanning electron microscope (SEM) analysis. This comparison revealed significant differences in the cell performance, which could be attributed to the amount and continuity of the residual strontium zirconate (SZO) interphase.