Gadolinia-doped Ceria Research Articles

Depth-direction analysis of nickel depletion in a Ni–gadolinia-doped ceria hydrogen electrode after steam electrolysis operation

We have examined the initial-stage degradation of a Ni–gadolinia-doped ceria (GDC) cermet hydrogen electrode prepared on a GDC buffer layer of a stabilized-zirconia electrolyte supported cell during steam electrolysis operation at 800 °C. With use of an air reference electrode, the IR-free electrode potential E and ohmic resistance of the hydrogen electrode side (RH2-side) were recorded. After a steam electrolysis operation at −1.0 A cm−2 for 211 h, the RH2-side increased appreciably, by 36 %, while the value of E was nearly unchanged. It was found via a depth-direction analysis with use of focused ion beam-scanning ion microscopy that the remaining percentage of Ni decreased to ca. 60 % in the layer between 1 and 3 µm from the top of the GDC buffer layer, followed by a further decrease to 42 % at 0.5 µm. Since this suggests a significant cathodic polarization within the thin reaction zone, an enlargement of the reaction zone, together with a stabilization of Ni contacting with GDC, could be essential to mitigate such a degradation.

Polarity‐induced grain growth of gadolinium‐doped ceria under field‐assisted sintering technology/spark plasma sintering (FAST/SPS) conditions

AbstractThis study aims to understand the effect of the electrical field on microstructure evolution during field‐assisted sintering or spark plasma sintering (FAST/SPS) of 10 mol% gadolinium‐doped ceria (GDC) with experimental and numerical methods. The novelty of this study has been the observation of enhanced grain growth in the region closer to the anode, even under FAST/SPS conditions with electrical fields less than 5 V/cm. The grain growth kinetics, including determination of activation energy and grain‐boundary mobility, were analyzed along the cross section of the samples for different temperatures and dwell periods. With an increase in distance from the anode, reduction in the activation energy for grain growth and grain‐boundary mobility was observed. These observations attributed to the attraction of oxygen ions to the anode region under an electrical field with an increase in defects along the grain boundaries. Thereby an increase in the grain‐boundary mobility and larger grains in that region were observed. A homogenous microstructure was observed in a case where the current did not flow through the sample. Furthermore, a numerical strategy has also been developed to simulate this behavior in addition to heat generation, heat transfer, and densification using Finite Element Methods (FEM) simulations. The simulation results provided an insight into the presence of a potential difference across the cross section of the samples. The simulation results were also in good agreement with the experimental observations.

Entropic stabilization plays a key role in the non-uniform distribution of oxygen ions and vacancy defects in gadolinium-doped ceria

Gadolinium-doped ceria (GDC) is an important fast oxygen ion conductor. O2--ion hopping in different cation environments of GDC is probed using molecular dynamics (MD). We find that at equilibrium, the O2--ions and vacancy defects arrange themselves around the relatively immobile cations in a way such that pairs of O2--ions and vacancy defect (O2--vac pair) are in greater numbers in some cation environments than others. The difference in O2--vac pair numbers is shown to originate from entropic effects in the cation environments. As a consequence of the entropic effect, the O2--vac pair distribution is practically independent of temperature between 974 and 1874 K. Even the O2- ion hopping rate and barrier are influenced by the entropic effect. This observation is in stark contrast with the standard belief that energetic interactions dominate in solid oxide materials, and entropic factors can be ignored. By analyzing the cation environment effect using k-means clustering, nine environment clusters are identified that are associated with unique values of the activation barrier and entropy. Comparisons to yttria stabilized zirconia (YSZ), yet another popular fast oxygen ion conductor, are made.

A nanoarchitectured cermet composite with extremely low Ni content for stable high-performance solid oxide fuel cells

ABSTRACT A strategy for improving the stability of nickel-based solid oxide fuel cell (SOFC) anodes via compositional and microstructural engineering is presented. Ni content was reduced to 2 vol%, and nanosized Ni particles were uniformly dispersed in a mixed ionic-electronic conducting matrix comprising gadolinium-doped ceria (GDC) using a thin-film technique. Remarkable stability with no performance deterioration even after 100 reduction-oxidation cycles could be observed for the optimized nanostructured anodes. Cell performance at 500°C was enhanced, exceeding 650 mW/cm2. This study offers valuable insights for enhancing the durability, performance, and productivity of SOFCs.

Isostatic pressing of screen printed nickel-gadolinium doped ceria anodes on electrolyte-supported solid oxide fuel cells

Microstructures of porous electrodes have great importance for promoting electrochemical reactions in solid oxide fuel cells (SOFCs). The present paper describes the application of isostatic pressing of screen printed nickel (Ni) gadolinium doped ceria (GDC) composite anode slurry on an electrolyte supported cell (ESC) and quantitatively discusses its influence on the cell electrochemical performance and microstructure. The current-voltage characteristics and electrochemical impedance spectra of the fabricated anodes of the ESC are measured. The microstructure parameters of the anodes are obtained by focused ion beam-scanning electron microscopy (FIB-SEM). The isostatic pressing decreases the porosity of the screen printed anodes, improves the Ni connectivity and increases the active triple phase boundary (TPB) density, but it does not change significantly the total TPB and double phase boundary (DPB) densities. Additionally, pressing enhances the adhesion of the electrode to the electrolyte. These observations are consistent with the improvement of the electrochemical performance of the isostatically pressed anodes.

Nickel–molybdenum catalysts for combined solid oxide fuel cell internal steam and dry reforming

A series of supported Ni-Mo catalysts were synthesised through a deposition-co-precipitation method (DcP) and investigated in combined steam and dry reforming. The prepared Ni-Mo catalysts supported by Yttria-Stabilised Zirconia (YSZ), Gadolinium-Doped Ceria (GDC) and Ceria Stabilised Zirconia (CSZ) with improved carbon tolerance have the potential to be used as anodes in Solid Oxide Fuel Cells (SOFC). These catalysts will be applied in the anode supporting layer where most of the internal reforming reaction takes place. The catalysts were compared to conventional reforming catalysts, where Al2O3 and TiO2 supports are typical for reforming catalysts. The catalytic performance of the Ni-Mo catalysts according to the support materials was observed in the sequence Al2O3 > YSZ > TiO2 > CSZ > GDC. Formation of Mo2C improved catalytic activity towards the steam reforming reaction. Increasing the Mo loading to Ni catalyst improved the coke resistance properties; however, it reduced the catalytic activity.

Ultra-Thin GDC Buffer Layer Fabricated By Vacuum Slurry Coating Method for High Performance Intermediate-Temperature SOFC

The Vacuum Slurry Coating (VSC) method has the advantage of being able to produce thin films with a simple device and is cost effective over other thin film coating methods. In solid oxide fuel cells (SOFCs), gadolinium doped ceria (GDC) buffer layer should be thin and dense to minimize ohmic resistance as well as prevent the undesirable reactions between scandia-stabilized zirconia (ScSZ) electrolyte and cobaltite perovskite cathodes. In this study, GDC buffer layer was coated on pre-sintered anode-supported half-cell comprising an ScSZ electrolyte layer on the NiO-YSZ anode using vacuum slurry coating process. Then GDC coated cells were sintered at 1400oC. The GDC buffer layer coated on the pre-sintered cell at 1200oC was thin and dense regardless of the immersion time. The maximum power density of the SOFC using VSC was 400 mW/cm2 (@ 700oC), which is 38 % higher than that using screen printing method (290 mW/cm2). The microstructure of the coated GDC buffer layer using the VSC method was much denser than using the screen printing method.

Photo-Enhanced Grain Boundary Ionic Conductivity in Gadolinium Doped Ceria

Oxide-based ionic and mixed ionic-electronic conductors (MIEC) play strategic roles in applications ranging from energy conversion and storage to thin film nano-ionic devices such as fuel cells, batteries, permeation membranes and nano-electronic memory and sensing devices. Most cost-effective production methods for fabricating both bulk and thin films samples generally result in the formation of polycrystalline microstructures with grain boundaries, where the crystallographic periodicity is interrupted.Grain boundaries possess significantly different properties compared to the bulk and often block ion migration. Due to the high resistance and large activation energies, they represent a ubiquitous problem to overcome for a variety of electrochemical energy storage and conversion devices especially for low temperature applications. Space-charge potential barriers, featuring depletion zones and band bending, forming at the interface between adjacent grains, have been used to explain their blocking nature in polycrystalline semiconductors.In the field of photoelectrochemistry, above band-gap light is well-known to contribute to reduce band bending at interfaces by providing additional charge carriers to screen potential barriers. Here, we show that the same principle can be applied to ionic conductors as well, and that we can significantly decrease the grain boundary impedance contribution in such systems.We illustrate photo-enhanced ionic conduction in a thin film of 3% Gd-doped ceria upon illumination by UV light, whose photon energy is slightly above ceria’s band-gap. In fact, we demonstrate the ability to decrease the grain boundary resistance by 72% at 250°C and reduce its activation energy nearly in half from 1.12 eV in the dark to 0.68 eV under UV light illumination. We further demonstrate that this effect is not caused by heat, electronic conductivity, surface catalysis or other effects. Our findings are based on electrochemical impedance spectroscopy (EIS) and intensity-modulated photocurrent spectroscopy (IMPS) measurements, performed on polycrystalline and epitaxial samples, and backed by theoretical considerations of grain boundary potential height and space charge regions.This work was supported by JSPS Core-to-Core Program, A. Advanced Research Networks: “Solid Oxide Interfaces for Faster Ion Transport”, Department of Energy, Basic Energy Sciences and the Swiss National Science Foundation. Figure 1

(Invited) High Performance La0.3Ca0.7Fe0.7Cr0.3O3-Δ-Based Cathodes for the Conversion of CO2 to CO in Solid Oxide Electrolysis Cells

Ni-based cathodes have been widely accepted as the state-of-the-art in terms of their superior catalytic activity for the CO2 reduction reaction (CO2RR) in solid oxide electrolysis cells (SOEC). However, they are prone to NiO formation and coking at high conversion rates, resulting in performance degradation and subsequent loss of activity (1, 2). For these reasons, numerous efforts have focused on investigating alternative electrocatalysts to replace Ni-based SOEC cathodes. Perovskite oxides, with a general formula ABO3, are a versatile class of materials that have very good catalytic properties that can be tailored by doping a variety of cations on the A- or B-site of the parent lattice. Extensive efforts in our group have been focussed on a very promising class of perovskite electrocatalysts, namely La0.3M0.7Fe0.7Cr0.3O3-δ (M = Sr, Ca) (LMFCr) that are not only highly active for oxygen reduction/evolution, but also for CO2 reduction (3-5). These materials are mixed ionic electronic conductors (MIEC) that also exhibit excellent compatibility with conventional electrolyte materials and hence can be employed directly as electrode layers without the need for composite electrodes that require mixing with ionic conductors, e.g., gadolinia-doped ceria (GDC) etc. Futhermore, the same electrocatalyst can be used as both the fuel and oxygen electrode which enables the cell to run reversibly (reversible solid oxide fuel cells, RSOFC). This is highly advantageous in terms of simplifying cell design and hence lowering cost, while also mitigating compatibility issues with neighboring cell components and generating long term stability (6), thus making the LMFCr electrocatalyst very promising for commercialization purposes. Of the two LMFCr analogues, La0.3Ca0.7Fe0.7Cr0.3O3-δ (LCFCr) exhibits superior catalytic activity for both the CO2RR and OER and therefore it is the focus of this work. Here, the stability of the LCFCr catalyst under high CO2 conversion to CO has been examined, determining whether these conditions lead to any deleterious effects, such as coke formation. We have answered this question by studying the stability and electrochemical performance of LCFCr in various CO2:CO environments. Furthermore, we have also employed Ni-doped LCFCr to decorate the perovskite surface with in situ exsolved metal nanoparticles (NP) to determine if this would further enhance its catalytic activity and stability. The LCFCr electrode ink was prepared by employing a 1:1 ratio of powder to ink vehicle. Symmetrical cells were prepared by tape casting the LCFCr ink over a ca. 0.50 cm2 area on both sides of 2.5 cm diameter SDC-buffered YSZ/SSZ substrates. Au paste was painted on as the current collector followed by sealing the cells on an alumina tube with a ceramic sealant. The fuel electrode was then exposed to various CO2:CO ratios (100:0, 90:10, 70:30, and 50:50), while air was supplied to the oxygen electrode at flow rates of 50 ml/min. The electrochemical performance of the cells was determined via electrochemical impedance spectroscopy (EIS) (at a 50 mV ac amplitude and over a range of 65000 – 0.01 Hz) and using cyclic voltammetry (CV at 5 mV/s). The short- and medium-term stability of the cells was determined by monitoring the current at various cell voltages of 1.0 – 1.6 V. The cells showed excellent performance for CO2RR in these gas mixtures, while the CO oxidation activity plateaued as the overpotential was increased. The CO2RR and CO oxidation activity was significantly enhanced when Ni-doped LCFCr with in situ exsolved Fe-Ni alloy NPs was employed as the electrode layers, also showing remarkably stable performance over several hours at 1.2 V without any evidence of coking or delamination up to 90% CO2 conversion, as revealed by postmortem SEM studies. The CO2/CO cells were also studied using synchrotron radiation at the Canadian Light Source in order to determine whether any changes occurred to the LMFCr surface or bulk properties and if any carbon could be detected, especially at very high conversion of CO2 to CO.References Yue and J. T. S. Irvine, Journal of The Electrochemical Society, 159, F442 (2012)Song, Z. Zhou, X. Zhang, Y. Zhou, H. Gong, H. Lv, Q. Liu, G. Wang and X. Bao, Journal of Materials Chemistry A, 6, 13661 (2018)K. Addo, B. Molero-Sanchez, M. Chen, S. Paulson and V. Birss, Fuel Cells, 15, 689 (2015)Molero-Sanchez, J. Prado-Gonjal, D. Avila-Brande, M. Chen, E. Moran and V. Birss, International Journal of Hydrogen Energy, 40, 1902 (2015)Haag, B. Madsen, S. Barnett and K. Poeppelmeier, Electrochemical and Solid State Letters, 11, B51 (2008)A. Hauch, S. D. Ebbesen, S. H. Jensen and M. Mogensen, Journal of Materials Chemistry, 18, 2331 (2008)

High Performance Commercial SOFC Cathodes Achieved with YSZ Nanoparticles

The electrochemical performance of solid oxide fuel cell (SOFC) cathodes was improved via integration of ultra-high surface area ceramic nanoparticles, up to 115 m2·g-1, generated with a novel processing method herein introduced. The processing method generates the high surface area nanoparticles at traditional SOFC sintering temperatures in two steps. In the first step, a hybrid inorganic-organic material comprising the ceramic precursors is sintered in an inert atmosphere at any temperature between 850°C-1350°C. During this step, an amorphous carbon template is generated in situ, preventing coarsening of the ceramic nanoparticles. The carbon template is then removed during the second step by a mild calcination in air at 700°C. Yttria-stabilized zirconia (YSZ), lanthanum strontium cobalt ferrite (LSCF), gadolinium-doped ceria (GDC), and strontium titanate (STO) have all been successfully prepared by this method. YSZ nanoparticles (nYSZ) were incorporated into a lanthanum strontium manganite-YSZ (LSM-YSZ) cathode of a commercial cell, resulting in a 90% increase in maximum power density. Impedance spectroscopy indicates the large improvement in power density was attributed to a combination of significant improvement in polarization resistance, a 45% decrease, and ohmic resistance, a 35% decrease. Remarkably, the performance of the cell modified with nYSZ was higher than cells modified with two mixed ionic electronic conductors (MIEC): PrxBa1-xCo3-δ (PBC) and LaxSr1-xCoyFe1-yO3-δ (LSCF). Both MIECs enhanced the density of active sites, but, unlike nYSZ, did not significantly improve ohmic resistance. The results demonstrate a novel pathway to achieve high performance in commercial SOFCs.

Electrochemical Characterization of Pr(A)MnO3 Perovskites As Oxygen Electrodes for Solid Oxide Fuel-Assisted Electrolysis Cells

Long-term degradation remains the main issue for the viability of solid oxide electrolysis cell (SOEC) technology as a practical hydrogen production system. The major specific degradation mechanism in SOECs relates to delamination phenomena at or near electrolyte/anode interface. The principle of so-called fuel-assisted electrolysis is to supply the carbon-containing species which can react with oxygen at the anode side thus bringing down the oxygen chemical potential at the electrolyte/anode interface and improving its stability. The present work is aimed at the characterization of PrMnO3-based perovskites for potential application as anodes in solid oxide fuel-assisted electrolysis cells.Pr0.6-x A0.4MnO3 ±δ (A = Sr, Ca; x = 0 and 0.05) powders were synthesized by glycine-nitrate combustion technique. The characterization included XRD, SEM/EDS, XPS, dilatometry and thermogravimetry, measurements of electrical properties, and determination of oxygen nonstoichiometry by coulometric titration. XRD analysis confirmed the formation of solid solutions with orthorhombic perovskite-like structure. Pr0.6-x A0.4MnO3 ±δ exhibit negligible variations of oxygen content with temperature and oxygen partial pressure under oxidizing conditions, while reducing p(O2) below 10-4 atm results in a progressive oxygen losses from the lattice and reduction of variable-valence cations. The low-p(O2) stability boundary of the perovskite phase at 800°C corresponds to ~10-17-3×10-16 atm; the stability domain is wider for Ca-substituted compositions and narrows with introduction of A-site vacancies. Dilatometric studies confirmed a good thermomechanical compatibility with common solid electrolytes. The electrical conductivity of Pr0.6-x A0.4MnO3 ±δ ceramics is p-type electronic and decreases with reducing p(O2), but still exceeds 40-50 S/cm under oxygen chemical potentials anticipated for the oxygen electrode operation conditions at 800°C. The electrochemical performance of Pr0.6-x A0.4MnO3 ±δ porous electrode layers was evaluated in contact with yttria-stabilized zirconia solid electrolyte using symmetrical cell configuration as function of relevant parameters (electrode fabrication conditions, with and without ceria-based interface layers, modifications via impregnations by praseodymia or gadolinia-doped ceria).

Feasibility Synthesis and Characterisation of Gadolinia Doped Ceria Coatings Obtained By Cathodic Arc Evaporation

The SOFC/SOEC community is now convinced that it is necessary to reduce the operating temperature of fuel cells to make the systems more robust, more reliable and thus to consider large-scale development. At the material level, the change of the reference electrolyte (YSZ) by a cerium compounds allows a significant performance gain for lower temperatures of use. It is obviously necessary to keep in mind the constraints related to the electrolyte application in solid electrolyte device, namely: good ionic conductivity unlike the electrical conductivity which must be low, electrochemical compatibility with the electrodes, be dense and covering on porous substrates.In this study, Gadolinia doped ceria coatings were deposited by cathodic arc evaporation from a metallic Ce-Gd (90/10 at%) target inserted into a conventional multiarc Ti evaporation target in the presence of a reactive argon-oxygen gas mixture. The structural and chemical features of these films were determined by X-ray diffraction and scanning electron microscopy. Their electrical properties were characterised using impedance spectroscopy measurements. It is shown that the as-deposited coatings crystallise in the fluorite type fcc structure of ceria and that their composition is the same than that of the target. The morphology of the coatings is influenced by the evaporation parameter (stress and droplet). The electrical measurements showed two contributions in Nyquist representation (figure1) and the activation energy is slightly higher than that given in the literature data for the bulk material.Figure 1 : Nyquist impedance plots of GDC coating, registered at 400°C, for two a.c. signal amplitudes. Logarithms of frequencies are indicated in the figure. Figure 1

Scale up of Proton-Conducting Ceramics into Multi-Cell Stacks

Proton-conducting ceramics are emerging as an enabling material for efficient electricity generation, energy storage, and fuels synthesis. While recent advancements at the lab-scale are highly encouraging, there are few reports of scaling cell size beyond the button cell, and no demonstrations of multi-cell stacks. Through support from the U.S. Department of Energy, researchers at the Colorado School of Mines are scaling up protonic-ceramic devices from the button-cell level into small, multi-cell stacks as illustrated in the figure. Both fuel-cell and electrolyzer stacks are in development. The order-of-magnitude increases in the physical size of the delicate membrane-electrode assembly (MEA) bring concerns regarding mechanical strength and robustness. The compatibility of protonic-ceramic materials with stack-packaging materials – metallic interconnects, current collectors, glass-ceramic sealants, and gaskets – has witnessed limited study. In this presentation, we describe selection and tuning of materials, fabrication procedures, and operating conditions to achieve reasonable performance and low degradation in protonic-ceramic fuel cells (PCFCs) stacks.We find highest stack electrochemical performance and durability when utilizing electrolytes based on BaCe0.4Zr0.4Y0.1Yb0.1O3-d (BCZYYb). The anode support is a nickel-electrolyte composite, while the cathode is BaCe0.4Fe0.4Zr0.1Y0.1O3-d (BCFZY). The planar MEAs reach 5 cm2 in active area. The circular stack design enables a measure of flexibility in MEA physical size and shape, and provides balance in stress distribution from thermal- and chemical-expansion. The MEAs are packaged within ferritic-steel interconnects and macor frames to form multi-cell stacks. Our three-cell stack demonstrates encouraging performance, reaching 0.69 and 0.47 W cm-2 under H2 and CH4 fuels, respectively, at 600 ºC. A cathode-electrolyte interlayer of 10%-gadolinium-doped ceria proves critical in achieving stack degradation as low as 1.5% kh-1. Figure 1

In operando x-ray photoelectron spectroscopy studies of H2 oxidation and H2O electrolysis on gadolinia-doped ceria electrodes

In operando, ambient-pressure x-ray photoelectron spectroscopy (AP-XPS) has been used to evaluate surface states of gadolinia-doped ceria (GDC) thin-film electrodes during H2 oxidation and H2O electrolysis, on yttria-stabilized zirconia (YSZ)-supported solid oxide electrochemical cells (SOCs). Porous nickel (Ni) and gold (Au) overlayers deposited on separate GDC thin films served as current collectors and potential electrocatalysts to facilitate heterogeneous chemistry for H2 oxidation or H2O electrolysis. Electrochemical characterization of the GDC thin-film electrodes complemented in operando XPS measurements of the O 1s spectra to correlate electrochemical overpotentials with surface chemistry near the Ni/GDC and Au/GDC interfaces. Shifts in O 1s binding energies across the metal/GDC/YSZ interfaces signified changes of local surface potential and provided a means of estimating kinetic parameters associated with charge transfer reactions. Effective oxygen partial pressure and surface potential impacted oxide vacancy and ceria polaron concentrations in the GDC, resulting in different reactivities of the GDC under the tested conditions. Both the Ni/GDC and Au/GDC demonstrated much higher currents for H2O electrolysis vs. H2 oxidation for comparable metal/GDC overpotentials due to increased electronic conductivity of the GDC under positive potentials and associated spreading of the electrochemically active region away from the triple-phase boundary. Higher electrochemical activity of the Ni/GDC electrode is attributed to the increased H2 activation on Ni in promoting charge transfer reactions (particularly for H2 oxidation). These results provide a basis for developing more informed reaction mechanisms for both H2 oxidation and H2O electrolysis of GDC-based composite electrodes in SOCs.

The fracture behavior of non-stoichiometric gadolinia-doped ceria solid electrolytes using molecular dynamics simulations

Gadolinia-doped ceria (GDC) electrolytes create non-stoichiometric oxygen vacancies and reduce mechanical properties under reducing conditions. Especially when there are crack defects in the material, an excess of oxygen vacancies create and propagate cracks to the point of ultimate failure of the electrolyte membrane. In this paper, we investigate the effect of crack defects on anisotropic mechanical behavior of non-stoichiometric GDC by using the molecular dynamics (MD) method. It is found that in the [100] and [110] crystal orientations, crack defects cause a change from ductile to brittle fracture. For non-stoichiometric GDC with low doping concentration, oxygen vacancies increase fracture stress by increasing critical nucleation stress of phase transformation. However, for non-stoichiometric GDC with high doping concentration, excess oxygen vacancies reduce the strength of fluorite phase. The fracture stress exhibits anomalous characteristics that first increases and then decreases with increasing oxygen vacancies. Due to the electrostatic repulsion between the adjacent oxygen ion layers in the [111] crystal orientation, the material strength decreases with increasing oxygen vacancies and crack defects.

Leveraging catalytic effects of heterointerfaces through multilayering for superior cathode performance

Cathode materials with high activity for oxygen surface exchange are required to achieve high-performance solid oxide fuel cells (SOFCs) at intermediate operating temperatures. LSC (La0.6Sr0.4CoO3-δ) and gadolinia-doped ceria (GDC, Ce0.9Gd0.1O1.95) thin films are alternately stacked to create multilayered cathode structures on yttria-stabilized zirconia (YSZ) electrolytes using pulsed laser deposition. Analysis of the electrochemical impedance spectra of symmetrical cells having varied number of LSC and GDC layers revealed that the existence of additional heterointerfaces leads to catalytic effects on the oxygen exchange properties. This is evidenced by a monotonic enhancement of the oxygen surface exchange coefficient (k*), commensurate with the increasing number of LSC-GDC interfaces. Due to the remarkable enhancement of the oxygen exchange properties, a significant lowering of the polarization resistance with increasing number of LSC-GDC interfaces is achieved. Furthermore, the LSC-GDC multilayers exhibited better long-term stability of the polarization resistance compared to a single-layer LSC in cell tests up to 600 h performed at 600 °C in air. These results highlight the important role of similar heterointerfaces in SOFC structures and provide guidance for the rational design of novel cathode materials with superior electrochemical performance at intermediate temperatures.

Surface analysis of lanthanum strontium cobalt oxides under cathodic polarization at high temperature through operando total-reflection X-ray absorption and X-ray fluorescence spectroscopy

The surface chemistry of perovskite-type lanthanum transition metal oxides is an important parameter in high-temperature electrochemical devices such as solid oxide fuel cells (SOFCs). Strontium segregation on the surface of lanthanum strontium cobalt oxides (LSC) may lead to poor oxygen reduction kinetics. However, previous studies have not sufficiently addressed the oxide surface chemistry, including the oxidation change of LSC under cathodic polarization. In this study, operando total-reflection X-ray fluorescence absorption and X-ray fluorescence spectroscopy were performed to investigate cathodic polarization at high temperature (773 K) and at 1 atm pressure. Dense thin-film electrodes of La0.6Sr0.4CoO3 (LSC113) on Zr0.92Y0.08O1.96 electrolytes with a gadolinium-doped ceria interlayer were examined as model SOFC cathodes. This may be the first time that predominant surface reduction of cobalt is directly observed by using LSC113 under cathodic polarization. This phenomenon is related to strontium enrichment on the LSC surface. (La0.5Sr0.5)2CoO4 decoration on LSC113 enhanced the oxygen reduction kinetics compared with that of bare LSC113. This is due to the suppression of cobalt surface reduction and Sr enrichment in the LSC113 phase.

Improvement in the Electrochemical Performance of Anode‐supported Solid Oxide Fuel Cells by Meso‐ and Nanoscale Structural Modifications

AbstractTo improve the electrochemical performance of anode‐supported solid oxide fuel cells (SOFCs), microextrusion printing and wet infiltration techniques are employed for structural modification on the meso‐ (10–100 µm) and nanoscale order, respectively. In the mesoscale structural modification, anode ridge structures are fabricated by extruding an anode slurry on the surface of a flat anode disk to extend the electrode–electrolyte interfacial area. In the nanoscale structural modification, gadolinium‐doped ceria (GDC) nanoparticles are introduced into a porous lanthanum strontium cobalt ferrite (LSCF) cathode. To investigate the effects of mesoscale and nanoscale structural modifications, four different types of anode‐supported SOFC including a conventional cell are prepared, and their performance is evaluated at several operating temperatures. It is found that both the mesoscale and nanoscale structural modifications reduce not only the polarization resistance but also the ohmic resistance in the cells, resulting in the improvement in cell performance. Moreover, it is clarified that the improvement in cell performance becomes greater with decreasing operating temperature. Specifically, the maximum power density in the cell where both mesoscale and nanoscale structural modifications are applied is increased by 66% at 600 °C and 34% at 700 °C, compared with that in the conventional cell.

Tailoring the ions and bandgaps in a novel semi-ionic energy conversion device for electrochemical performance

The new semi-ionic energy conversion (SIEC) device has attracted remarkable attention owing to its clean and environmentally friendly applications. In this device, novel materials and mechanisms have been explored using electronic and ionic conductor materials. The tuning effect of the ions and bandgap has been studied to investigate the structural, optical, and electrochemical performance of the material. Composite materials, gadolinium-doped ceria-cadmium-doped ZnO (GDC-ZnCdO), based on ionic gadolinium-doped ceria (GDC) and semiconductor (ZnCdO) in molar ratios of 1:4, 2:3, 3:2, and 4:1 have been prepared by a wet chemical route. The crystalline structure of the GDC-ZnCdO was studied and found to have cubic and hexagonal wurtzite phases with an average crystallite size of 30–40 nm. The morphology of the prepared composite materials is a homogenous and porous structure. It was found that the addition of GDC increases the transmittance and shows a red shift in the bandgap from 2.70 eV to 2.46 eV. The maximum conductivity of 2.0 S/cm1 was achieved for the sample 4GDC-1ZnCdO at 700°C. Electrochemical impedance spectra and X-ray photoelectron spectroscopy analysis were performed to investigate the electrochemical properties of the prepared semi-ionic composite materials. The SIEC device showed a much better performance than a conventional solid oxide fuel cell. The maximum open-circuit voltage (OCV) of about 1.013 Vand power density of 0.65 W/cm2 were obtained using hydrogen fuel at 600°C, as compared with a conventional fuel cell with 0.72 V and 0.27 W/cm2, respectively. Hence, the results reveal that the ions and bandgap tuning play a crucial role in fuel cell functions. Therefore, it has been determined that the bandgap can be tuned to obtain a better and more stable performance of the SIEC device. This study presents a novel approach to enhance the electrochemical performance with the tailoring of the new semi-ionic materials.

A Nanoarchitectured Cermet Composite with Extremely Low Ni Content for Stable High-Performance Solid Oxide Fuel Cells

A strategy for improving the stability of nickel-based solid oxide fuel cell (SOFC) anodes via compositional and microstructural engineering is presented. Ni content was reduced to 2 vol%, and nanosized Ni particles were uniformly dispersed in a mixed ionic-electronic conducting matrix comprising gadolinium-doped ceria (GDC) using a thin-film technique. Remarkable stability with no performance deterioration even after 100 reduction-oxidation cycles could be observed for the optimized nanostructured anodes. Cell performance at 500 °C was enhanced, exceeding 650 mW/cm 2 . This study offers valuable insights for enhancing the durability, performance, and productivity of SOFCs.