Porous GDC Research Articles

Enhanced performance of reversible solid oxide fuel cells by densification of Gd0.1Ce0.9O2−δ (GDC) barrier layer/La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF6428)-GDC oxygen-electrode interface

In reversible solid oxide cells (RSOCs) with yttria-stabilized zirconia (YSZ) electrolyte and LSCF6428-based oxygen electrode, a GDC barrier layer is used to prevent inter-diffusion of constituents. However, the performance of such RSOC is limited to the achievable density of GDC at prevailing temperatures of traditional fabrication methods. In this work, the GDC barrier layer was densified via infiltration with polyvinylpyrrolidone (PVP) chelated metal precursors sol and subsequent sintering at 1200 °C. The NiO-YSZ fuel electrode-supported RSOCs with YSZ electrolyte, porous or infiltrated GDC barrier layer and LSCF6428-GDC composite oxygen electrode were fabricated. The RSOCs were analyzed with respect to operating temperature (650–800 °C), water partial pressure at fuel electrode (pH2OFE=0.03–0.5 atm), oxygen partial pressure at oxygen electrode (pO2AE=0.02–0.21 atm) and current loading. The performance evaluation, using current-voltage curve and electrochemical impedance spectroscopy (EIS), showed that the RSOC with an infiltrated GDC barrier layer exhibited a 32–48% increase in performance in fuel cell (FC) mode in the 650–800 °C range, compared to that of the RSOC with a porous GDC layer. Similarly, in electrolysis cell (EC) mode, comparing current density at 1.3 V, chosen as the reference voltage for ease of comparison, based on a thermoneutral voltage of 1.285 V in electrolysis cell (EC) operation at 750 °C[1], the RSOC with an infiltrated GDC barrier layer exhibited values of over 1.2 A∙cm−2, in contrast to the RSOC with a porous GDC barrier layer, which showed 0.8 A∙cm−2. In the case of the RSOC with infiltrated GDC, a durability test was conducted at 750 °C under an EC current density of 1A∙cm−2for 100 h. Our experimental results demonstrate that the RSOC with an infiltrated GDC barrier layer not only exhibits enhanced performance but also maintains stability. Comparative analysis was conducted using Distribution of Relaxation Time (DRT) analysis of Electrochemical Impedance Spectroscopy (EIS) data, to further understand the differences between the cells.

Controlling oxygen defect chemistry at electrolyte surface of intermediate temperature solid oxide fuel cells

Engineering the anion defect, especially oxygen vacancy, at the electrolyte surface is important because the concentration and distribution of defects at this surface often determine the oxygen reduction reaction (ORR) kinetics and, in turn, the electrochemical performance of intermediate temperature solid oxide fuel cells (<700 °C). However, designing the desired defect structures is a challenging issue because the required concentration and distribution of defects strongly depend on the locations in the electrolyte with different functionalities. In this study, we precisely control the defect chemistry at the electrolyte surface (Gd0.1Ce0.9O2-δ, GDC) through the infiltration method while maintaining the surface structures. The porous GDC scaffold is modified by the conformal GDC layer in a thickness of 7–11 nm with a controlled trivalent dopant (Gd3+) ratio in the range of 0–40 mol%. The enriched oxygen vacancies at the electrolyte surface remarkably improve the ORR kinetics, providing extended and more active reaction sites. An optimized cell with the doping ratio of 30 mol% shows 1.81 times increased maximum power density of 1.07 Wcm−2 at 650 °C compared with a non-infiltrated cell.

Conducting Property and Performance Evaluation of LNF as Cathode Current Contact Layer in Solid Oxide Fuel Cell

Electrical performance, durability and degradations are challenging issues for Solid Oxide Fuel Cell (SOFC) commercialization. Ce0.9Gd0.1O2-δ (GDC) and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) are current gold material system in SOFC, which mainly works as barrier layer and cathode, respectively. However, the porous GDC barrier layer and vulnerable cathode/electrolyte interface often leads to high ohmic resistances and fast degradations. Herein, we develop and report a novel approach for barrier layer and cathode modifications, using cost-competitive and scalable hydrothermal reaction.Results show that, the as-sintered porous GDC barrier layer is filled up with in-situly grown Ceria based nano particles during hydrothermal reaction, yielding improved density and sintering-active surface, simultaneously. After co-sintered with cathode, the cell obtains optimized cathode/barrier layer interface, exhibiting decreased ohmic and cathode polarization resistances with higher electrical performance and less degradation.On the other aspect, the Ceria based nano particles are also observed in-situly precipitated on LSCF powder surface during hydrothermal reaction, forming dual-phase LSCF@GDC cathode, which also yields decreased ohmic and polarization resistance than pristine LSCF.Above results suggest that hydrothermal modifications are effective for barrier layer and cathode/electrolyte interface optimizations. Thus, the improved oxygen ion transfer and interface contact are beneficial for SOFC electrical performance and long-term operation. Furthermore, this hydrothermal modification is applicable in many other surface and interface optimizations.

Novel structured anode-supported solid oxide fuel cells with porous GDC interlayers

In this study, the effects of porous GDC interlayer(s) addition into NiO-YSZ anode functional layer on the performance of anode supported solid oxide fuel cells are experimentally studied. The number and porosity of GDC interlayer(s) are also considered in the study. For this purpose, firstly three different NiO-YSZ anode supported cells having various NiO-YSZ anode functional thicknesses are fabricated by tape casting and screen printing methods as base cases. Other cells are also fabricated similarly by adding 1–3 GDC interlayer(s) having different porosities into anode functional layer, keeping the total anode functional layer thickness as the same. The porosity of the interlayer is controlled by changing the pore former content in the tape casting solution of GDC interlayer from 5 wt % to 20 wt%. Thus, fifteen anode supported cells in total are fabricated and tested. All cells have 16 cm2 active area and are characterized by performance and impedance tests as well as microstructural analyses. The results reveal that it is possible to improve the cell performance by GDC interlayer addition. The base cell prepared with three NiO-YSZ anode functional layers provides only 0.283 Wcm-2 peak power density at 800 °C, while the modified cell, where NiO-YSZ layer in the middle is substituted by a GDC interlayer prepared with 15 wt % pore former, exhibits 0.582 Wcm-2 maximum power density at the same operating conditions. Furthermore, the cell performance is found to decrease with increasing the number of GDC interlayer, while it is shown to increase with the pore former content up to 15 wt %. The variations in the cell performance are due to changes in the ohmic, charge transfer and diffusion resistances as confirmed by impedance analyses as a result of resultant anode functional layer microstructure depending on the cell design observed by scanning electron microscope.

Electrical properties of gadolinia doped ceria electrolytes fabricated by infiltration aided sintering

Common solid oxide fuel cell (SOFC) electrolyte materials (e.g., gadolinia doped ceria – GDC) demand temperatures exceeding 1400 °C for densification by conventional solid state sintering. It is very desirable to reduce the densification of the SOFC electroltytes to i) avoid microstructural coarsening of the composite anode layers, which are co-sintered with the electolyte layer in the anode supported SOFC fabrication scheme and ii) reduce energy consumption during SOFC manufacturing. We have recently demostrated a novel infiltration-aided sintering route to densify GDC ceramics at 1200 °C. In the present work, we present the electrical properties of GDC ceramics fabricated thusly. Comparison of high density (≥95%) samples fabricated by conventional or infiltration-aided sintering reveal that at 700 °C, similar total electrical conductivities are obtained, while at 300 °C, specific grain boundary resistivity is smaller in the latter. Bulk (grain) conductivity is higher in porous GDC ceramics (relative density ≤ 90%) fabricated by infiltration-aided sintering than the conventionally sintered ones with similar porosities. Finally, open circuit voltage of 0.84 V at 700 °C, obtained under dilute hydrogen and stagnant air conditions suggests that GDC ceramics densified by infiltration-aided sintering are suitable for use as SOFC electrolytes.

Electrochemical performance of La0.6Sr0.4Co0.2Fe0.8O3–Ce0.9Gd0.1O2-δ composite SOFC cathodes fabricated by electrocatalyst and/or electrocatalyst-ionic conductor infiltration

Infiltration of electrocatalyst precursor solutions into previously sintered porous ionic conductor scaffolds has been used recently as an alternative method to the conventional co-sintering route to fabricate electrocatalyst-ionic conductor composites for solid oxide fuel cell (SOFC) cathode applications. However, the aqueous nitrate solutions generally used to perform the infiltration process results in electrocatalyst precipitates that are disconnected from each other, yielding poor electrode performance. In this work, polymeric electrocatalyst (La0.6Sr0.4Co0.2Fe0.8O3—LSCF) precursors that produce interconnected thin films upon heat treatment were used to infiltrate porous ionic conductor Ce0.9Gd0.1O2-δ (GDC) scaffolds to overcome these issues. In addition, for the first time in the literature, a mixture of LSCF and GDC polymeric precursors, which would yield LSCF–GDC nanocomposite coatings on the grains of the porous GDC scaffold were used as the infiltration solution. Thus, further enhancement of the electrocatalyst/ionic conductor interfacial area and achievement of improved electrode performance was aimed. As a result of the optimization studies, the lowest measured area specific polarization resistance (ASRcathode) values of 0.47 and 0.73 Ω.cm2 were obtained for polymeric LSCF+GDC and LSCF precursor infiltrations respectively at 700 °C, in air. In addition, LSCF+GDC infiltration yielded electrodes with much improved long-term stability in comparison to those obtained by LSCF infiltration.

Fabrication of Nanofibrous La1-XSrxCoO3/GDC Composite Cathode Using a Combination of Chemically Assisted Electrodeposition and Infiltration Techniques for Solid Oxide Fuel Cells

We developed a new method for the fabrication of La1-xSrxCoO3-GDC composite cathodes in a nanofibrous morphology combining chemically assisted electrodeposition and infiltration techniques for solid oxide fuel cells. Fabrication method of nanofibrous La1-xSrxCoO3-GDC composite cathode for solid oxide fuel cells consists of chemically assisted electrodeposition of La0.6CoO3 into a porous GDC scaffold followed by an infiltration of Sr to produce a La1-xSrxCoO3 phase after a heat treatment at 900 °C. The fabrication method for La1-xSrxCoO3-GDC composite cathode involves following steps: fabrication of a highly porous GDC scaffold on a dense ScCeSZ electrolyte of an anode supported button cell by screen printing a GDC ink containing 30 wt % PMMA pore former and then sintering at 1300 °C; infiltration of 0.5 M cobalt nitrate solution into porous GDC scaffold to act as seed for CNT growth by catalytic chemical vapor deposition of carbon 750 °C using ethylene as carbon source; deposition of La0.6CoO3 by chemically assisted electrodeposition of La and Co from an aqueous electrolyte containing La(NO3)3-6H2O and Co(NO3)2-6H2O; heat treatment of the electrodeposited GDC scaffold is in air at 750 °C for 5 h to remove the CNT; infiltration of strontium and heat treatment at 900 °C to form the La1-xSrxCoO3 phase. A detailed schematic of the fabrication of nanofibrous La1-xSrxCoO3-GDC composite cathode is shown in figure 1. The new fabrication method for nanofibrous La1-xSrxCoO3-GDC composite cathode uses a low fabrication temperature (900 °C) as compared to the conventional sintering which may avoids the formation of secondary phases with ScCeSZ electrolyte. The new fabrication method for nanofibrous La1-xSrxCoO3-GDC composite cathode also provides excellent control over La1-xSrxCoO3 loading and a desired loading can be achieved in a single electrodeposition step. Nanofibrous morphology of new cathode can provide large surface area and can also assist in transport of oxidant through nanoporous structure. The complete anode supported button cells produced by the new fabrication technique have shown a high electrochemical performance (613 mW cm-2 at 750 °C) while using humidified hydrogen as fuel

Reversible operation of microtubular solid oxide cells using La0.6Sr0.4Co0.2Fe0.8O3-δ-Ce0.9Gd0.1O2-δ oxygen electrodes

Yttria stabilized zirconia (YSZ) based microtubular solid oxide fuel cells (mT-SOFCs) using La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) and Ce0.9Gd0.1O2-δ (GDC) as the oxygen electrode, along with a porous GDC electrolyte-electrode barrier layer, were fabricated and characterized in both fuel cell (SOFC) and electrolysis (SOEC) operation modes. The cells were anode-supported, the NiO-YSZ microtubular supports being made by Powder Extrusion Moulding (PEM). The cells showed power densities of 695 mW cm−2 at 800 °C and 0.7 V in SOFC mode, and of 845 mA cm−2 at 800 °C and 1.3 V in SOEC mode. AC impedance experiments performed under different potential loads demonstrated the reversibility of the cells. These results showed that these cells, prepared with a method suitable for using on an industrial scale, are highly reproducible and reliable, as well as very competitive as reversible SOFC-SOEC devices operating at intermediate temperatures.

Thermodynamic perspective of Sr‐related degradation issues in SOFCs

Abstract(La,Sr)(Co,Fe)O3‐δ is very common as cathode material in SOFC applications. Sr in this type of cathode material is very reactive to form secondary phases with other oxides, which affect micro‐structures and properties of the cathode materials, GDC layers and ZrO2‐based electrolytes. The Sr‐related degradation issues, Cr poisoning and volatile Sr species formation, are studied. As supplement to existing experimental knowledge on Cr poisoning, specific thermodynamic aspects for Cr poisoning are discussed. The thermodynamic calculations show that the partial pressure pCrO3 has a stronger temperature dependence than pCrO2(OH)2, and when considering the reaction between SrO and CrO3(g), dependent on different pCrO3 and pO2, different Sr–Cr–O compounds SrCrO4, SrCrO3, Sr3Cr2O8 or Sr2CrO4 could be formed. In addition, thermodynamic calculations show that in the presence of water vapor, formation of volatile Sr(OH)2 is possible as well. pSr(OH)2 depends on temperature, pH2O and SrO activity and can be of the same order of magnitude as pCrO2(OH)2. Volatile Sr(OH)2 can diffuse through the porous GDC layer and react with ZrO2‐based electrolytes to form SrZrO3 precipitates. The reaction between gaseous Sr species and 8YSZ sheet is studied experimentally. The surface of the 8YSZ sheet is investigated by SEM coupled with EDS, confirming the deposition of Sr.

TEM and ETEM Study on SrZrO3 Formation at the LSCF/GDC/YSZ Interfaces

Transmission electron microscopic (TEM) studies have elucidated SrZrO3 formation and growth at La0.6Sr0.4Co0.2Fe0.8O3 (LSCF)/ Ce0.9Gd0.1O2 (GDC) / yttria-stabilized zirconia (YSZ) interfaces in the solid oxide fuel cell (SOFC). SrZrO3 forms firstly at the GDC/YSZ interface: at the interface between GDC dense layer and YSZ, at grain boundaries in the GDC dense layer and on the surface of the GDC dense layer during sintering LSCF at 1100 oC. Then, SrZrO3 grows to the both sides of YSZ electrolyte and porous GDC layer. Electron energy loss spectroscopy revealed the inter-diffusion of La, Ce and Gd as well as Sr and Zr in the vicinity of the GDC/YSZ interface. La is solute in SrZrO3, and SrZrO3 shows tetragonal crystal structure, which has the double pseudocubic perovskite subcell. Nucleation and growth of SrZrO3 orthorhombic phase was observed at 800 oC under an O2 atmosphere in an environmental TEM.

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.

Anode fabrication for solid oxide fuel cells: Electroless and electrodeposition of nickel and silver into doped ceria scaffolds

A novel fabrication method using electroless and electrodeposited Ni/Ag/GDC for SOFC anodes is presented. First a porous Ce0.9Gd0.1O2−x (GDC) scaffold was deposited on a YSZ electrolyte by screen printing and sintering. The scaffold was then metallized with silver using Tollens' reaction, followed by electrodeposition of nickel from a Watt's bath. The electrodes (Ni/Ag/GDC) were tested in both symmetrical and fuel cell configurations. The microstructures of the Ni/Ag/GDC anodes were analyzed using scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX). Nano-particles of Ni formed in the porous GDC scaffold provided triple phase boundaries (TPB). The electronic conductivity of the Ni/Ag/GDC (3.5/24.7/71.8 vol%) electrode was good even at relatively low Ni volume fractions. The electrochemical performance was examined in different concentrations of humidified hydrogen (3% H2O) and over a range of temperatures (600–750 °C). The total area specific resistance (ASR) of the anode at 750 °C in humidified 97 vol% H2 was 1.12 Ω cm2, with low-frequency polarization (R_l) as the largest contributor. The electrodes were successfully integrated into a fuel cell and operated in both H2 and syngas.

Preparation and characterization of Pr2NiO4+δ infiltrated into Gd-doped ceria as SOFC cathode

Composite electrodes of Pr2NiO4+δ and Gd-doped ceria (Ce0.8Gd0.2O1.90 so-called GDC) have been prepared starting from a Pr-Ni nitrate solution (2:1 stoichiometry ratio) that was infiltrated into a porous GDC backbone made by screen printing onto 3 % yttria-stabilized zirconia membrane (90 μm thick). The crystallization of the Pr2NiO4+δ powder was first studied by X-ray diffraction: it highly depends on the annealing atmosphere (air or N2) and the formation of this phase as infiltrate is more difficult than for the powder. The electrochemical study performed on symmetrical cells shows that the polarization resistance decreases when the annealing temperature of the infiltrate electrode decreases. The lowest R p value is obtained for an annealing at 900 °C, for 2 h in N2, the value being 0.075 Ω cm2 at 600 °C in air.

(Invited) Intrinsic Oxygen Surface Exchange Resistance Measurements on Mixed Ionic Electronic Conducting Infiltrate Nanoparticles

Introduction Over the past decade, the Solid Oxide Fuel Cell community has developed high-performance Nano-Composite Cathodes (NCCs) by infiltrating, gelling, and thermally decomposing mixed ionic electronic conducting (MIEC) nitrate precursor solutions inside porous ion-conducting (IC) scaffolds.1, 2 Unfortunately, attempts to identify optimal infiltrate compositions and predict optimal NCC microstructures has been complicated by 1) a lack of intrinsic oxygen surface exchange resistance (RS ) values for infiltrated MIEC nano-particles, and 2) the large RS variations observed in the literature for bulk and thin film MIEC samples with nominally identical compositions. The present work addresses these problems by using NCC polarization resistance (RP ) measurements and Finite Element Modeling (FEM) of 3D Focused Ion Beam- Scanning Electron Microscopy (FIB-SEM) reconstructions to back-calculate the RS values for a variety of infiltrated MIEC nano-particle compositions. Experimental Methods Following the procedures described in Burye and Nicholas,3-5 La0.6Sr0.4FeO3-x (LSF)-Ce0.9Gd0.1O1.95 (GDC), La0.6Sr0.4Fe0.8Co0.2O3-x (LSFC)-GDC, La0.6Sr0.4Co0.8Fe0.2O3-x (LSCF)-GDC, and La0.6Sr0.4CoO3-x (LSC)-GDC NCCs were prepared through multiple nitrate solution infiltrations into porous GDC scaffolds using either a standard (S), desiccation (D) or nano-GDC pre-infiltration (P) approach. Open-circuit 500-700oC NCC electrochemical impedance spectroscopy RP measurements in air were then taken, and the infiltrate surface areas were determined via SEM. 3D NCC microstructural reconstructions were obtained by epoxy infiltrating non-infiltrated GDC scaffolds in epoxy, using the FIB-SEM to collect 2D serial sections, and using the MIMICS computer program (Materialise) to produce a 3D finite element meshes of the cathode and electrolyte. Following the procedures discussed in Nicholas and Barnett,6 open-circuit NCC RP calculations for various RS and infiltrate surface area combinations were performed by placing an arbitrary voltage across the reconstructed volume (1 V) and solving Laplace’s Equation inside the GDC scaffold. Infiltrate RS values were then identified as those producing the observed RP values with a given, experimentally-measured infiltrate surface area. Results As an example, Figure 1 shows how the LSFC infiltrate RS values compare with bulk and thin film values reported in the literature.7-16 Conclusions All the MIEC infiltrate compositions tested here exhibited RS values that were within the range of RS values and activation energies previously reported in the literature. Analyses also confirmed that just as in bulk and thin film studies, the infiltrated RS values for LSC<LSCF<LSFC<LSF. Acknowledgements This work was supported by National Science Foundation award number CBET-1254453.

The Effect of Precusor Gel Desiccation, Ceria Oxide Pre-Infiltration, and Precursor Solution Additives on Infiltrated Lanthanum Strontium Ferrite-Lanthanum Strontium Cobaltite Solid Solution Nano-Particles

Here, average La0.6Sr0.4CoxFe1-xO3-δ (0≤x≤1) nano-particle sizes were controllably modulated from 48 to 24 nm by desiccating La0.6Sr0.4CoxFe1-xO3-δ (0≤x≤1) precursor nitrate solutions or infiltrating La0.6Sr0.4CoxFe1-xO3-δ (0≤x≤1) precursor nitrate solutions into Ce0.9Gd0.1O1.95 (GDC) scaffolds pre-infiltrated with a large amount of GDC nano-particles. Scanning electron microscopy, thermo-gravimetric analysis, and x-ray diffraction experiments conducted on decomposed/decomposing precursor gels within, and outside of, porous GDC scaffolds showed that desiccation and pre-infiltrated nano-GDC resulted in reduced nitrate decomposition temperatures and reduced La0.6Sr0.4CoxFe1-xO3-δ (0≤x≤1) average particle sizes. GDC scaffolds infiltrated with La0.6Sr0.4CoxFe1-xO3-δ (0≤x≤1) precursor gels, desiccated with CaCl2, and subsequently fired at 700oC, contained high-phase purity La0.6Sr0.4CoxFe1-xO3-δ (0≤x≤1) nano-particles with average particle sizes ranging from 31-27 nm. GDC scaffolds pre-infiltrated with nano-GDC infiltrated particles, infiltrated with La0.6Sr0.4CoxFe1-xO3-δ (0≤x≤1) precursor gels and subsequently fired at 700oC, contained high-phase purity La0.6Sr0.4CoxFe1-xO3-δ (0≤x≤1) nano-particles with average particle sizes ranging from 29-24 nm. In contrast to comparable, standard nano-composite cathodes (NCC’s) containing 48 nm average-sized La0.6Sr0.4CoxFe1-xO3-δ (0≤x≤1) particles and achieving polarization resistances of 0.10 Ωcm2 at 640°C, the best CaCl2-dessicated NCC’s achieved 0.10 Ωcm2 at 600°C, while the best pre-infiltrated nano-GDC NCC’s achieved 0.10 Ωcm2 at 540°C. Acknowledgements This work was supported by National Science Foundation award number CBET-1254453. Figure 1. Scanning electron micrographs for La0.6Sr0.4CoxFe1-xO3-δ nano-particles produced inside a GDC scaffold at 700°C for citric acid-containing precursor gels that were standard (a-e) desiccated (f-j) and pre-infiltrated nano-GDC oxide particles (k-o). Note, LSF = La0.6Sr0.4FeO3-x, LSFC = La0.6Sr0.4Fe0.8Co0.2O3-x, LSCF55 = La0.6Sr0.4Fe0.5Co0.5O3-x, LSCF = La0.6Sr0.4Co0.8Fe0.2O3-x, and LSC = La0.6Sr0.4CoO3-x. All scale bars are 50 nm in width. Figure 1

Electrochemical response of several cathode configurations prepared with Ba0.5Sr0.5Co0.8Fe0.2O3-δ and Ce0.9Gd0.1O1.95 for IT-SOFC

The electrochemical response of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) electrodes prepared by an acetic acid-based gel route has been investigated by impedance spectroscopy (IS) as a function of temperature (400 ≤ T ≤ 900 °C) and oxygen partial pressure (1 × 10−3 ≤ pO2 ≤ 1 atm). Several electrode configurations were studied using Ce0.9Gd0.1O1.95 (GDC) as the electrolyte. These consisted of one BSCF layer (cell A), a BSCF layer with an intermediate porous GDC layer (cell B), and graded electrodes using a composite BSCF+GDC, with variations in the surface area of GDC (cells C and D). The optimum heat treatment for the electrode assemblages was determined to be around 850–900 °C. Analysis of the impedance spectra shows that at T ≥ 600 °C a low frequency (LF) contribution, associated to the gas phase diffusion is systematically the rate-limiting step. All the electrodes show an intermediate frequency (IF) arc related to mixed processes. For cells A and B, the IF response is related to the oxide ion transfer at the electrode/electrolyte surface and the charge transfer at the electrode surface, while for cells C and D the mixed process involves the charge transfer and the molecular oxygen dissociation at the electrode surface.

Performance of La0.8Sr0.2Co0.8Ni0.2O3-δ-Based Oxygen Electrode for Solid Oxide Electrolysis Cells

Solid oxide electrolysis cell (SOEC), considered as the reverse reaction of solid oxide fuel cell (SOFC), is expected as a highly efficient way to produce hydrogen environmentally friendly. The reduction of operating temperature and enhancement of the durability are now vital requirements for industrial commercialization of SOEC. The cost of the system can be reduced by reducing the operating temperature from the traditional 1000◦C to 600-800◦C. La0.8Sr0.2Co0.8Ni0.2O3-δoxides (LSCN) were selected as innovative mixed electronic and ionic conductivity (MIEC) oxygen electrode materials because of their high electrical conductivity and electrocatalytic activity with low thermal expansion coefficient (TEC)[1]. The biggest degradation issue for SOEC is the delamination of the oxygen electrode from electrolyte [2]. In the present work, we fabricated two kinds of LSCN-based oxygen electrodes to improve the structural stability. The solution impregnated La0.8Sr0.2Co0.8Ni0.2 + Gd doped CeO2(LSCN+GDC) oxygen electrode and LSCN-GDC composite oxygen electrode based on Ni-YSZ hydrogen electrode-support button cells are fabricated to assess their performance and the stability. For LSCN-GDC composite oxygen electrode, on the one hand, GDC was added for its high oxygen ion mobility at intermediate temperature and mechanically and chemically compatible with LSCN. On the other hand, the graded LSCN and GDC powder can form a porous continuous structure to improve the mechanical stability. GDC layer was used for making a barrier between LSCN-GDC composite oxygen electrode and YSZ electrolyte to avoid interfacial reaction [3]. For the impregnated LSCN+GDC oxygen electrode, distributing the LSCN nanoparticles on the porous GDC scaffold can transfer the reaction sites from the electrode/electrolyte interface to the LSCN nanoparticles/GDC grains interface in the bulk of the electrode. Before and after electrolysis mode at constant operating conditions, current-voltage (I-V) tests and Electrochemical Impedance Spectroscopy (EIS) were measured and compared to trace any increase in ohmic and polarization resistance of the cells. Besides, the stability of the button cell was studied in galvanostatic SOEC operation. Results show that the impregnated LSCN+GDC composite oxygen electrode exhibited lower ohmic resistance for the high conductivity provided by continuous LSCN nanoparticles and comparable polarization resistance at 800◦C before the electrolysis, and more durable during the SOEC operation. Post-experimental analyses were conducted using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) to characterize the changes of microstructure and phase after operations, especially the interface condition of the oxygen electrode and electrolyte, to investigate and compare the degradation mechanisms. [1] X. Xu, F.Z. Wang, Y.H. Liu, J. Pu, B. Chi and J. Li. J. Power Sources, 196, 9365(2011). [2] M.A. Laguna-Bercero. J. Power Sources, 203, 4(2012). [3] J. Chen, F.L. Liang, L. Liu, S.P. Jiang and J. Li. J. Hydrogen Energy, 34, 6845(2009).

Surface Modification of LaNi0.6Fe0.4O3-δ Film Electrode by Ce0.9Gd0.1O1.95 Porous Layer

1. Introduction Our previous study on the composite electrode between mixed ionic and electronic conductor (MIEC) of LaNi0.6Fe0.4O3-δ (LNF) and ion-conducting oxide of Ce0.9Gd0.1O1.95(GDC) showed high electrochemical performance [1]. The lack of understanding on reaction mechanism makes the enhancement of composite electrode always connected to the enhancement of active region length of the electrode. However, for electrode which has quite high ionic conductivity such as LNF, another factor may enhance as well [2]. The surface oxygen exchange rate may enhance due to the hetero junction effect with GDC. To prove our hypothesis, LNF film electrode was prepared and GDC porous layer added on top of the surface of the film. In this study, the effect of the GDC was analyzed and discussed. 2. Experimental A LNF film electrode was prepared by pulse laser deposition method by Nd-YAG laser on the GDC electrolyte. Porous LNF as a counter electrode was deposited on the opposite side to the LNF film on the GDC electrolyte. The GDC porous layer was deposited into top of the film surface by screen printing method. The diameter of GDC porous layer was smaller than LNF film electrode. The samples were calcined at 1273 K for 6 hours. Electrochemical measurement was taken to compare two samples between LNF film and LNF film-GDC porous layer as a function of Temperature (973 – 1073 K) and oxygen partial pressure (10-3-1 bar). For the impedance set up, the alumina ring which was covered with Pt mesh was used as a current collector. The diameter of the alumina ring was larger compared to the GDC layer that the alumina ring didn’t touch GDC. In the analyses of the impedance responses, tentative equivalent circuits with two or three R-CPE elements (a resistance, R, and a constant phase element, CPE, connected in parallel) were used to identify their physical meanings. Secondary ion mass spectrometry measurement (SIMS) was also performed to determine the surface oxygen exchange kinetics of the samples. The samples with LNF film and LNF film – GDC porous layer were cut into pieces of 3 mm x 3 mm. The samples were pre-annealed at 973 K under 10-1 bar for 3 hours and then quenched to room temperature. The isotope exchange was performed by flowing 16O2 at 973 K under 10-1 bar for 30 minutes before the gas abruptly changed into 18O2for 10 minutes. The quenched samples after the isotope exchange were used for SIMS measurement. For LNF film – GDC porous sample, the GDC porous layer was removed from LNF film prior to SIMS measurement. The isotope depth profile technique was used to determine the surface exchange kinetics for both of the samples. 3. Results and Discussion From electrochemical impedance spectroscopy (EIS) result, the depressed semicircle of impedance spectra was observed for the LNF film and LNF film – GDC porous electrode. The low frequency arc was related to the electrode reaction on both of the samples for which the area specific conductivity was calculated. Fig. 1 shows the comparison of area specific conductivity (ASC) of LNF film and LNF film - GDC porous. The LNF film - GDC porous layer has higher ASC compared to the LNF film electrode. It shows that the activity of oxygen reduction process enhances due to the GDC existence on the top of LNF film surface. The ASC which was estimated from SIMS were close compared to the EIS result as shown in Fig. 1. The area specific conductivity which was estimated from k*by the following equation [3]: σe= (k*.2.n.F 2.Co )/(R.T) Where the F, and Co are faraday number and oxygen concentration in the bulk sample. The n (between 2 to 4) is dependent on the reaction kinetics. Two measurements by EIS and SIMS proved that GDC on the surface of LNF film enhances the surface oxygen kinetics. Within the surface modification by porous GDC, the ASC enhances significantly. It may be due to the GDC on the LNF gives the oxygen vacancy at the interface between GDC and LNF film surface. The oxygen vacancy from GDC increases the rate of oxygen reduction reaction on the electrode surface. At the current result, the solid conclusion on the mechanism remains unclear. Reference [1] R. A. Budiman et al., ECS Trams., 57(2013) 1873. [2] M. Nishi et al., ECS Trans., 45, 171 (2012). [3] T.Kawada et al., Solid State Ionic, 121 (1999) 271. Figure 1

Synthesis of Porous Submicron Gd0.1Ce0.9O1.95Particles by Carbon Nanoparticle-Addition Ultrasonic Spray Pyrolysis (CNA-USP) and Nanoparticle Preparation by Ball Milling

A new ultrasonic spray pyrolysis method, called carbon nanoparticle-addition ultrasonic spray pyrolysis (CNA-USP), is developed to synthesize nanoparticles of electrolyte material for solid oxide fuel cell applications. In CNA-USP, carbon nanoparticles are added in a precursor solution. First, Gd0.1Ce0.9O1.95 (GDC) particles were synthesized from an aqueous solution of Ce(NO3)3 6H2O and Gd(NO3)3 6H2O by using the CNA-USP method. The resulting synthesized GDC particles were agglomerated, porous, primary particles on the order of 10 nm in diameter. EDX images revealed uniform distributions of Ce, Gd, and O in these porous particles. Then, these agglomerated, porous submicron GDC particles were ground into primary nanoparticles by ball milling for 24 h. The average diameter of the ground GDC nanoparticles was about double of their average crystallite size.Copyright 2014 American Association for Aerosol Research

Tailoring Mixed Ionic Electronic Conducting Nano-Particle Size through Desiccation and/or Doped Ceria Oxide Pre-Infiltration

Introduction Mixed Ionic and Electronic Conducting (MIEC) multi-cation oxide nano-particles have been used to enhance the performance of SOFC electrodes [1]. These SOFC MIEC nano-particles are often produced by thermally decomposing gelled MIEC precursor solutions infiltrated into porous ionic conducting (IC) scaffolds. Average SOFC nano-particle sizes produced by this thermal decomposition technique are typically 55 nm in diameter [1, 2], and result in SOFC operating temperatures (defined as the lowest temperature where the electrode polarization resistance, (RP), is equal to 0.1 Ωcm2) in excess of 600°C. Since MIEC particle size directly correlates with the surface area available for oxygen incorporation (and hence SOFC electrode performance), the objective of this study was to determine whether it was possible to reduce MIEC nano-particle size through desiccation and/or sequential infiltration. Experimental Methods Heavily-infiltrated nano-composite La0.6Sr0.4Co0.8Fe0.2O3 (LSCF) on Ce0.9Gd0.1O1.95 (GDC) and LSCF-GDC on GDC nano-composite cathodes were prepared through multiple nitrate solution infiltrations into porous GDC scaffolds. For the LSCF on GDC samples, precursor LSCF solutions were gelled inside a GDC scaffold, desiccated, and fired at 700°C. For the LSCF-GDC on GDC samples, GDC precursor solutions were gelled inside a GDC scaffold and fired at 700°C prior to LSCF precursor solution infiltration, gelation, and firing at 700°C. Results As shown in Figure 1, average nano-particle sizes of 20 nm ± 5 nm (determined through scanning electron microscopy) were achieved through both desiccation (using CaCl2 as a desiccant) and sequential infiltration (using 8.0 vol % of pre-existing nano-GDC as a decomposition catalyst). By altering the amount of desiccation it is possible to controllably adjust the LSCF nano-particle size from 20 nm ± 5 nm ( note the ± refers to the standard deviation of the actual particle sizes and is not a reflection of the scanning electron microscopy (SEM) resolution, which is ± 1 nm) to 60 nm ± 18 nm. Similarly, sequential infiltration can be used to adjust average nano-particle size from 20 nm ± 5 nm to 50 nm ± 18 nm. As shown in Figures 2 & 3, the desiccation and sequential infiltration approaches progressively change precursor solution decomposition temperature peak positions. Figure 2 shows that desiccation shifts the strontium nitrate peak position to lower temperatures with stronger desiccants. Figure 3 demonstrates that as the nano-GDC loading level increases, the decomposition peak positions shift from 800°C with 0 vol % nano-GDC to 200°C with 50 vol % nano-GDC. (Volume percentage is the volume % of the IC scaffold porosity occupied by nano-particles). Through the adjustment of precursor gel decomposition peak position to lower temperatures smaller nano-particle sizes are achieved.As shown in Figure 4, NCCs infiltrated with LSCF with 20 nm nano-particle diameters have RP values of 0.1 Ωcm2 at 565°C using drying/desiccation and NCCs using LSCF + GDC with 20 nm nano-particle diameters have RP values of 0.1 Ωcm2 at 540°C using sequential infiltration. Conclusions Two different approaches have been developed to influence precursor solution decomposition temperature and reduce average nano-particle sizes. Average MIEC nano-particle size were reduced to 20 nm and SOFC operating temperatures (i.e. the temperature at which RP ≤ 0.1 Ωcm2) are 565°C with desiccation and 540°C with sequential infiltration. Acknowledgements This work was made possible through a Michigan State University faculty startup grant.