Pure GDC Research Articles

Enhanced sintering ability and electrochemical performance of Gd0.1Ce0.9O1.95 composited with (Dy0.2Zr0.05Bi0.75)2O3 for low-temperature solid oxide fuel cells

By compositing cerium oxide (Gd0.1Ce0.9O1.95, GDC) with the dysprosium-zirconium co-doped bismuth oxide (Dy0.2Zr0.05Bi0.75)2O3, or DZSB, a novel DZSB-GDC composite electrolyte is developed, which has excellent sintering ability and ionic conductivity. Reverse coprecipitation is used to prepare nano composite electrolyte powders, and the sintering and electrical properties with varying DZSB composite ratio are examined. After sintered at 1200 °C for 10 h, all composite electrolyte pellets can achieve densities of more than 95 %. Furthermore, as the DZSB ratio increases, the densification rate increases and the starting temperature of the densification stage decreases. 10DZSB-GDC shows the highest conductivity (σt) and has an activation energy of 0.92 eV below 550 °C, which is 8.9 % lower than that of pure GDC. At 400 °C, the grain boundary conductivity of 10DZSB-GDC was 113.8 times that of pure GDC. The ionic conductivity of 10DZSB-GDC remain stable at 0.015 S cm−1 at 600 °C for 4200 h without any decay. The open-circuit voltage and peak power density of 10DZSB-GDC based cell are 0.914 V, and 441.1 mW cm−2 at 750 °C, which is 1.1 and 2.1 times that of GDC-supported cells, respectively. This indicates that 10DZSB-GDC is a promising electrolyte material for low-temperature solid oxide fuel cells.

Ionic Conduction of Ceramic/Molten Salt Composite Electrolyte in Fuel Cell with Lithium Compound as Electrode

The ionic conduction of GDC electrolyte in a ceramic fuel cell (CFC) with NCAL as symmetrical electrode was investigated. Under the test condition of hydrogen concentration cell with pure hydrogen and 3% H2 + 97% Ar flowing into both sides of the cell, the open circuit voltage (OCV) of the cell with NCAL as the symmetrical electrode reaches 57 mV at 550 °C, while the OCV of the cell using Pt symmetrical electrode is close to 0 V，indicating that there is proton conduction in the cell with NCAL as the symmetrical electrode. The characterization results of SEM, XPS and TOF-SIMS indicate that the internal phase composition of the electrolyte of the as-prepared cell changes from pure GDC to GDC/LiOH + Li2CO3 + Li2O composite after the test of hydrogen concentration cell. Combined with the composition of the GDC/LiOH + Li2CO3 + Li2O composite electrolyte and the variation law of proton transference numbers at different temperatures, it is speculated that the carriers of the composite electrolyte formed during the test of CFC using NCAL symmetrical electrode in H2 may include OH− and/or O2− in addition to proton.

Excellent kinetics of single-phase Gd-doped ceria fuel electrodes in solid oxide cells

Solid oxide fuel electrodes with pure GDC as the functional layer exhibit excellent kinetics, in line with mechanistic impedance modelling.

Semantic Segmentation of SOFC Composite Electrode Images Incorporating Patch-Based Convolutional Neural Networks

Quantitative microstructure analysis is an important task in many fields of material engineering. In solid oxide fuel cell (SOFC), the microstructure of porous electrodes determines electrochemical performance of the cell. Three dimensional analysis by focused ion beam - scanning electron microscopy (FIB-SEM) has been successfully implemented to evaluate the fabrication strategies and to identify degradation mechanisms. For precise evaluation of the electrode microstructures, segmentation of large number of SEM images is required. The conventional method for image processing of raw FIB-SEM dataset consists of semi-automatic filtering, thresholding, edge-detection, etc. Those semi-automatic methods usually require manual corrections in final stages of the processing to ensure highest accuracy, which greatly extends the processing time. Therefore, number of images, size and resolution of the obtained microstructures are limited not only by the physical capabilities of FIB-SEM hardware, but also by the post processing of datasets.In the present research, the framework for automated SOFC 3D microstructure reconstruction form a large dataset of FIB-SEM images is proposed. The framework is based on machine learning, namely convolutional neural networks (CNN). A simple yet effective deep neural network incorporating the patch-based convolutional layers (patch-CNN) organized in the encoder-decoder manner was designed and tested. Clear guidelines are given for the selection and labeling of training images, selection of network architecture, methods for the CNN training from a scratch and transfer learning. The most influential factors to assure short processing time and accurate segmentation are the selection of optimal training dataset and the proper network architecture. Teaching images have to be labelized prior to training, since minimizing their number greatly reduces the preprocessing time. Selections of optimal network architecture and patch size are crucial in providing enough spatial information and maintaining reasonable training time. The data augmentation and random patch extraction allow artificial enlargement of the training dataset. The augmentation techniques, including brightness and contrast random enhancements, rotation and reflection, are discussed in relation to the original features of different microstructures.The proposed methodology was tested for various electrodes including nickel - yttria-stabilized zirconia (Ni-YSZ) anode, nickel - gadolinium doped ceria pattern electrode (Ni-GDC), pure GDC and La0.9Sr0.1Cr0.5Mn0.5O3-δ - Gd0.1Ce0.9O2-δ (LSCM-GDC) nano-composite porous anodes. The tested data had various resolutions (5 nm – 30 nm), different image sizes (500 – 4000 pixels) and had very different measurements artifacts. The CNN-based approach succeeded in accurate binarization, ternarization and quaternarization of micro- and nano-structures as well as impurities detection. The achieved pixel-based accuracy was over 96%, and the 3D microstructural parameters such as phase fractions, connectivity, total and active TPB densities, surface area density and tortuosity factors calculated from ground truth data and CNN-prediction were identical.One of the challenges of image segmentation in handling FIB-SEM image set is the treatment of artifacts such as curtaining, shadows and charge-ups. Proposed patch-CNN can successfully process all of these artifacts. The detection of pores which are not filled with resin prior to the FIB-SEM measurement is another difficult task for the conventional image processing. The results achieved for the samples with a large number of un-filled pores are presented in Fig. 1. Reconstruction of this sample was not possible with the conventional algorithms based on the pixel brightness values. The two separated CNNs were trained to conduct artificial pore-filling operation (CNN1) and to ternarize the phases (CNN2). The amount of labelized data incorporated for training was 3 and 17 images for CNN1 and CNN2, respectively. The final segmentation was conducted for 935 images and the reconstructed volume size was 54 × 54 × 28 µm with resolution of 30 nm in all spatial directions.Deploying artificial intelligence methods into real problems in material engineering requires fairness, high accuracy, transferability and interpretability. The proposed automatic patch-CNN algorithm can significantly shorten the image processing time with maintaining accuracy, resolution and size of the reconstructed microstructures, which were not available by conventional methods. Proposed algorithm can be easily extended for any multiphase, porous materials and different 3-D imagining techniques.Fig. 1 Framework for automated SOFC 3D microstructure reconstruction form a large dataset of FIB-SEM images without pore filling. Figure 1

Innovative improvement of sintered ceramic electrolytes by salt infiltration

Previously sintered (1500 °C, 4 h) dense pellets of Ce0.9Gd0.1O1.95 (GDC) were covered and heat treated with eutectic mixtures of Na2CO3 and Li2CO3 (NLC), and their electrical performance was assessed against pure GDC and chemically synthesized GDC + NLC. Microstructural analysis of NLC impregnated samples confirmed slight migration of the molten phase to the interior of the GDC pellets via grain boundaries, resulting in a significant improvement of the grain boundary conductivity, increasing with duration of heat treatment (0.5–2 h) and temperature (600–800 °C range). The observed total conductivity exceeded in almost 20% the corresponding values obtained for standard GDC samples. Cells tested before and after direct current polarization (0.5 V, 500 °C) showed the same electrical performance, discarding the possibility of parallel contributions of salt ions to the total conductivity. Grain boundary engineering using salt infiltration is an effective tool to improve the electrical performance of ceramic electrolytes.

Improving Grain Boundary Conductivity of Ce0.9Gd0.1O2 − δ Electrolyte through Compositing with Carbonate or Semiconductor

In the solid oxide fuel cell field, heterogeneous ion doping is a common methodology to improve the ionic conductivity of electrolytes, but overwhelming grain boundary resistance is still the main obstacle for low‐temperature applications. According to previous reports, building rapid ion transport at the grain boundary through compositing methods was considered as a proposed design for electrolytes to decrease the grain boundary resistance and obtain high ionic conductivity. Herein, Ce0.9Gd0.1O2 − δ (GDC) is selected as the matrix material and composited with Na2CO3 and NdBa0.5Sr0.5Cu2O5 + δ (NBSCu) to form GDC‐Na2CO3 nanocomposite and GDC‐NBSCu composite. The grain boundary conductivity (σb) is delicately separated from the EIS results, demonstrating that the σb of GDC‐NBSCu and GDC‐Na2CO3 composite are both substantially higher than that of pure GDC. Therefore, the power density maximum of GDC‐NBSCu and GDC‐Na2CO3 electrolyte is 726 mW cm−2 at 600 °C and 797 mW cm−2 at 575 °C, respectively. Variety of characterization reveales that the proton contributes to the enhancement of σb in GDC‐Na2CO3, while the band energy alignment between GDC and NBSCu works as an accelerator to promote the ionic conduction for GDC‐NBSCu.

Evaluation of the Microstructure and the Electrochemical Properties of Ce0.8(1−x)Gd0.2(1−x)CuxO[1.9(1−x)+x] Electrolytes for IT-SOFCs

The influence of copper addition (0.5–2 mol%) on the crystal structure, densification microstructure, and electrochemical properties of Ce0.8Gd0.2O1.9 synthesized in a one-step sol–gel combustion synthesis route has been studied. It has been found that Cu is very active as sintering aids, with a significative reduction of GDC firing temperature. A reduction of 500 °C with a small amount of copper (0.5 mol%) was observed achieving dense bodies with considerable ionic conductivities. Rietveld refined was used to investigate the crystal structure while relative density and microstructural examination were performed in the sintering temperature range of 1000–1200 °C after dilatometer analysis. High dense bodies were fabricated at the lowest sintering temperature, which promotes the formation of Ce0.8(1−x)Gd0.2(1−x)CuxO[1.9(1−x)+x] solid solution and the absence of secondary phase Cu-rich or the segregation or copper at the grain boundary. As compared to the pure GDC an improvement of total conductivity was achieved with a maximum for the highest copper content of 2.23·10−3–9.19·10−2 S cm−1 in the temperature range of 200–800 °C.

Performance of GDC with alkali metal carbonates as sintering aids

Gd-doped ceria electrolytes (GDC, 10 at.% Gd) were prepared using a eutectic mixture of Li and Na carbonates as sintering aid (NLC - 0, 1, 3 and 5 mol%), using two processing routes (ceramic and chemical). Structural and microstructural characterization provided no evidence for significant solubility of alkali metal cations in GDC. Materials sintered at 1100 °C/4 h were similar in densification (>95%) and microstructure to those obtained from a commercial powder sintered at 1500 °C/4 h. Impedance spectroscopy in air (200–600 °C) and as a function of oxygen partial pressure (from 0.21 to about 10−25 atm, 600–750 °C) showed that the low temperature conductivity of materials prepared using chemical synthesis surpassed the conductivity of a commercial GDC due to higher grain boundary conductivity. The estimated n-type conductivity of materials with NLC was also slightly higher than for pure GDC. The role of sintering aid and processing routes is discussed taking into consideration decomposition and volatilization of alkali salts.

Microstructure and electrical conductivity of Ce0.9Gd0.1O1.95-MgO composite electrolyte

Ce0.9Gd0.1O1.95 (GDC)-xMgO (x = 0, 10%, 20%, 30%, 40%, 50 mol%) powders were synthesized by sol-gel method and then the corresponding composite ceramics were sintered in air atmosphere at 1500 °C for 5 h. The sintered samples consist of both GDC and MgO phases on the condition of x ≥ 10%, and the MgO grains segregate at the grain boundaries of GDC. As the MgO content rises from 0 to 20%, the average grain size of GDC decreases from 2.15 to 0.41 μm. The introduction of MgO can improve the specific grain boundary electrical conductivity of GDC-xMgO composite ceramics dramatically. The GDC-20%MgO shows the highest specific grain boundary conductivity, which is 1.726 × 10−5 S/cm at 400 °C, 9.16 times higher than that of pure GDC. The variation in the real total conductivity confirms that the formation of GDC/MgO heterogeneous interfaces is beneficial to the enhancement in the electrical conduction properties of GDC-xMgO composite ceramics.

Electronic conductivity in Gd-doped ceria with salt additions

Samples of Gd-doped ceria (GDC, Ce0.9Gd0.1O1.95) with eutectic mixtures of Na2CO3 and Li2CO3 (NLC), or K2CO3, Na2CO3 and Li2CO3 (KNLC) as sintering aids (5 mol%) were studied against pure GDC and cobalt doped GDC. The former electrolytes reached densifications in excess of 95% when sintered at 1100 °C, values comparable to pure GDC sintered at 1500 °C, and more than 20% higher than for GDC sintered at 1100 °C. X-ray diffraction powder patterns showed only typical GDC peaks without any noticeable lattice modification and microstructural analysis suggested a homogeneous distribution of residual amounts of sintering aids. The electrical properties of all materials were studied by impedance spectroscopy in air from 200 to 750 °C. Materials with NLC and KNLC showed a total ionic conductivity matching pure GDC at 600 °C. Hebb-Wagner polarization measurements, used to assess the impact of sintering aids on the n and p-type electronic conductivity of GDC (600–750 °C) showed that NLC and KNLC additions lower the p-type conductivity of GDC while Co additions have the opposite effect. The efficacy of alkali metal salts to produce ceria-based electrolytes with competitive ionic and electronic transport properties is exposed.

Enhanced Oxygen Electrocatalysis in Heterostructured Ceria Electrolytes for Intermediate-Temperature Solid Oxide Fuel Cells.

Heterostructured composite ceria electrolytes have been shown to accelerate the oxygen reduction activity and provide a new approach to improve solid oxide fuel cell (SOFC) performance. In this study, barium carbonate was added to gadolinium-doped ceria, Gd0.2Ce0.8O2−δ (GDC) electrolyte to improve the electrochemical performance of intermediate-temperature SOFCs. The heterostructured electrolyte was formed by the addition of 5 wt % BaCO3 to a GDC electrolyte, resulting in a reaction during sintering that formed well-dispersed BaCe0.8Gd0.2O3−δ (BCG) throughout the electrolyte. The resulting material was tested as an electrolyte using La0.6Sr0.4Co0.2Fe0.8O3−δ as a cathode, resulting in a dramatic reduction to the polarization resistance of more than half the value (600 and 700 °C, the resistance was reduced from 2.49 and 0.23 Ω cm2 to 1.21 and 0.12 Ω cm2) obtained by using pure GDC as an electrolyte. Furthermore, full cell SOFC tests employing the heterostructured electrolyte conducted during overextended durations indicated that the BCG phase in the 5BCG–GDC electrolyte was stable in an air atmosphere with no observed reactions with residual CO2. This approach of tailoring surface reactivity by tailoring the composition and structure of the electrolyte as opposed to electrode materials provides an alternative method to improve fuel cell performance.

Gadolinium doped ceria interlayers for Solid Oxide Fuel Cells cathodes: Enhanced reactivity with sintering aids (Li, Cu, Zn), and improved densification by infiltration

This paper reports the study of the densification of 20% Gd doped ceria (Ce0.8Gd0.2O1.9 (GDC)) interlayers in SOFC cathodes through two different routes: the well-known addition of sintering elements, and an innovative densification process by infiltration. First, Li, Cu, and Zn nitrates were added to GDC powders. The effect of these additives on the densification was studied by dilatometry on pellets, and show a large decrease of the sintering temperature from 1330 °C (pure GDC), down to 1080 °C, 950 °C, and 930 °C for Zn, Cu, and Li addition, respectively. However, this promising result does not apply to screen-printed layers, which are more porous than pellets and in which the shrinkage is constrained by the substrate. The second approach consists in preparing a pre-sintered GDC layer, which is subsequently infiltrated with Ce and Gd nitrates and sintered at 1250 °C to increase its density. Such an approach results in highly dense GDC interlayers. Using La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) as electrode, the influence of the interlayers on the cathode performance was studied. The addition of sintering aids dramatically increases the cell resistances, most likely because the additives increase the reactivity between GDC and either Yttria Stabilized Zirconia (YSZ) or LSCF, thus losing the expected benefit related to the decrease of sintering temperatures. The interlayers prepared by infiltration do not induce additional resistances in the cell, which results in power densities of single cells 40–50% higher than those of cells prepared with commercial GDC interlayers, making this approach a valuable alternative to sintering aids.

Numerical Simulation of LSCF-GDC Composite Cathodes with Various Microstructures

In the present study, the electrochemical performances of different LSCF-GDC microstructures are numerically calculated in order to investigate the effects of GDC particles and GDC pillars on LSCF cathode microstructures with various particle sizes. Lattice Boltzmann method (LBM) is applied for solving the governing equations. It is found from the numerical results of pure GDC and LSCF-GDC composite cathodes with various particle sizes that GDC addition improves LSCF cathode electrochemical performance. This performance improvement is more pronounced for cathodes with smaller particle sizes and when operated at lower temperatures. In addition, it is shown that LSCF cathode with GDC pillars can enhance the performance of the cathode. The GDC pillars are even more effective when the reactive region of the cathode becomes thin by using small size powders.

Numerical Simulation of LSCF-GDC Composite Cathodes with Various Microstructures

In the present study, electrochemical performances of different LSCF-GDC microstructures are numerically simulated in order to investigate the effects of GDC particle sizes and GDC pillars on LSCF cathode performance. Lattice Boltzmann method (LBM) is applied for solving the governing equations. It is found from the numerical results of pure GDC and LSCF-GDC composite cathodes with various particle sizes that the performance improvement by GDC addition is more pronounced for cathodes with smaller particle sizes. In addition, it is shown that LSCF cathode with GDC pillars can enhance the performance of the cathode especially when the reactive region of the cathode becomes thin by using small size powders.

Effects of Electrolytes on Electrochemical Properties of Cathode Material LaNi0.6Fe0.4O3-d based Fuel Cells

Unique electrochemical properties of LaNi0.6Fe0.4O3-δ (LNF) have made it a better choice of cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs) with high power density. To improve electrochemical properties of LNF and analyze the effects of different electrolytes on the performance of LNF-based IT-SOFC, half-cells are fabricated with Ce0.9Gd0.1O2 (GDC) electrolyte or porous GDC backbone/GDC electrolyte. Ag paste is used as a current collector deposited on the cathode material surface. Additionally, LNF and GDC powders are synthesized by sol-gel method leading to the polarization resistance (Rp) and current density of half-cells with two kinds of electrolytes to be measured by a three-electrode method. Experimental results at 550-750 oC show that the polarization resistance (Rp) of LNF/GDC backbone/GDC is 0.376 Ω·cm2 at 750 oC in air, a reduction by 67.02 % compared with that of pure GDC electrolyte at the same temperature. And LNF/GDC backbone/GDC has a higher current density, 0.377 A·cm-2 at 750 oC in air. Systematic analysis suggests that porous GDC backbone improves electrochemical properties of LNF cathode materials.

TbxCe0.95-XGd0.05O2-δ (0.15 ≤ x ≤ 0.40) Cathode Materials Prepared through Solid State Route for Low Temperature SOFC

Four compositions of TbxCe0.95-xGd0.05O2-δ (0.15 ≤ x ≤ 0.40) have been prepared by varying the Tb content. Phase formation, thermal expansion, ionic conductivity, electronic conductivity, ionic transference number and optical properties have been investigated. XRD results indicate that TbxCe0.95-xGd0.05O2-δ (TCGO) samples crystallize in the fluorite structure. The coefficient of thermal expansion decreases with increasing x, and at x= 0.2 shows comparable TEC value of (12.8 x 10-6 K-1) with that of Ce0.80Gd0.20O2 (GDC) electrolyte at 50-800 °C temperature range. A maximum electrical conductivity of 0.008 S cm-1 is exhibited by the composition, x = 0.40 at 550 °C. Gd3+ contributes to ionic conduction by creating oxygen vacancies and Tb4+ contributes to electronic conduction by decreasing the band gap of CeO2. UV-Visible spectroscopy results show a band gap of 3.19 eV for pure GDC and 1.85 eV for the composition Tb0.20Ce0.75Gd0.05O2-δ.

The electrical properties of microwave sintered gadolinia doped ceria–alumina nano-composite electrolyte

Gadolinia doped ceria–alumina (GDC–Al 2O 3) nano composites have been prepared by chemical synthesis route and sintered using 2.45 GHz microwave energy as well as conventional technique. The electrical properties of the sintered samples are investigated by ac impedance spectroscopy. It has been observed that the conductivity of microwave sintered GDC–Al 2O 3 composite is higher than that of conventional sintered GDC–Al 2O 3 composite and pure GDC sample. Higher concentration of vacancies at the interfaces of GDC and Al 2O 3 phases may be responsible for the better conductivity of GDC–Al 2O 3 composite compared to pure GDC. The fine grain microstructure of microwave sintered samples creates more interfaces compared to conventional sintered sample which in turn responsible for the better performance of microwave sintered composite sample. The micro-structural results of microwave sintered samples also reveal the presence of elongated, needle like Al 2O 3 grains. The X-ray diffraction results have shown the formation of additional GdAlO 3 phase.

Effects of transition metal oxides on the densification of thin-film GDC electrolyte and on the performance of intermediate-temperature SOFC

Transition metal oxides (FeO 1.5 and CoO) were added to Gd-doped ceria (Gd 0.1Ce 0.9O 2−δ, GDC) powder for preparing the thin-film electrolyte used in the anode-supported intermediate-temperature solid oxide fuel cell (SOFC). NiO–GDC anode substrate in a weight ratio of 65:35 was fabricated by the tape-casting method. Thin-film electrolyte was fabricated on the pre-sintered anode substrate by screen-printing method and then co-sintered to form the electrolyte/anode bilayer. The cathode, which is made of La 0.6Sr 0.4Fe 0.8Co 0.2O 3 and GDC (LSCF–GDC) in a weight ratio of 50:50, was screen-printed on the thus-prepared electrolyte surface and sintered to form a complete single cell. The effects of transition metal oxides on the densification of thin-film GDC electrolyte and on the performance of intermediate-temperature SOFC were studied. Results showed that the densification temperature of thin-film GDC electrolyte could not be further reduced by modifying it with transition metal oxides (FeO 1.5 and CoO) as sintering aids. Both the addition of Fe and Co to GDC enhanced the p-type conductivity of the electrolyte resulting in decreased ohmic resistance. However, they played different effects on the polarization behavior of the cells. Fe-loading decreased the single cell polarization resistance, thus greatly enhancing the charge-transfer process below 600 °C. At 500 °C, the charge-transfer resistance of the single cell with Fe-loaded GDC electrolyte is only 78% of that of the cell with pure GDC electrolyte. Conversely, Co-loading inhibited the charge-transfer process in the whole testing temperature range. Thus, it can be concluded that Fe-loaded GDC electrolyte is a promising electrolyte material for intermediate- and low-temperature SOFC.

IR-assisted Sol-gel Synthesis of Ce0.8Gd0.2O2-δ for SOFCs Applications

Gadolinia-doped ceria (Ce0.8Gd0.2O2-δ, GDC) is an alternative electrolyte for Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFC). Nanostructured materials have recently attracted extensive attention because of their high reactivity that allowed milder sintering conditions. Although several methods were proposed for the synthesis of ultrafine powders, most of them generally require a subsequent calcination step. The aim of this work was to produce ultrafine, pure GDC powders using a standard chemical route (aqueous sol-gel) coupled with non-conventional heating processes (infrared heating). Nanocrystalline non-agglomerated GDC particles were successfully produced under mild conditions (170{degree sign}C, 1h) without the intermediate decomposition and/or calcining steps. Moreover the GDC obtained through the optimization of the IR-assisted sol-gel synthesis parameters exhibited values of ionic conductivity higher than the ones showed by commercial and conventional sol-gel produced powders of similar composition.