Highly Performing Chromate-Based Ceramic Anodes (Y0.7Ca0.3Cr1

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Functional Inorganic Materials and Devices

Highly Performing Chromate-based Ceramic Anodes (Y0.7Ca0.3Cr1xCuxO3-#) for Low-Temperature Solid Oxide Fuel Cells A. Mohammed Hussain, Ke-Ji Pan, Yi-Lin Huang, Ian Alexander Robinson, Colin Gore, and Eric D. Wachsman ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07987 • Publication Date (Web): 26 Sep 2018 Downloaded from http://pubs.acs.org on September 26, 2018

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ACS Applied Materials & Interfaces

Highly Performing Chromate-Based Ceramic Anodes (Y0.7Ca0.3Cr1-xCuxO3-δ) for LowTemperature Solid Oxide Fuel Cells A. Mohammed Hussain†,‡,ƚ, Ke-Ji Pan†,‡,↕, Yi-Lin Huang†,‡, Ian A. Robinson†,‡, Colin Gore‡,↕ and Eric D. Wachsman†,‡,* †

‡

Maryland Energy Innovation Institute, University of Maryland, College Park, MD, 20742 Department of Materials Science & Engineering, University of Maryland, College Park, MD, 20742 ƚ

Nissan Technical Centre North America, Advanced Materials and Technology Research, Farmington Hills, MI 48331 ↕

Redox Power Systems LLC, College Park, MD 20742

Keywords: Ceramic anode, low-temperature, Ni-GDC(Ce0.9Gd0.1O2-δ) nanoparticles, LSC(La0.6Sr0.4CoO3-δ) nanoparticles, cathode-supported SOFCs.

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Abstract Exploitation of alternative anode materials for low-temperature solid oxide fuel cells (LT-SOFCs, 350-650°C) is technologically important but remains a major challenge. Here we report a potential ceramic anode Y0.7Ca0.3Cr1-xCuxO3-δ (x=0, 0.05, 0.12, and 0.20) (YCC) exhibiting relatively high conductivity at low temperatures (≤650°C) both in fuel and oxidant gas conditions. Additionally, the newly developed composition (YCC12) is structurally stable in reducing and oxidizing gas conditions, indicating its suitability for SOFC anodes. The I–V characteristics and performance of the

ceramic

anode

infiltrated

with

Ni-(Ce0.9Gd0.1O2-δ)(GDC) was

determined

using

GDC/(La0.6Sr0.4CoO3-δ)(LSC) based cathode supported SOFCs. High peak power densities of ~1.2 W/cm2 (2.2A/cm2), 1 W/cm2 (2.0A/cm2) and 0.6 W/cm2 (1.3 A/cm2) were obtained at 600, 550, and 500 °C, respectively in H2/3% H2O as fuel and air as oxidant. SOFCs showed excellent stability with a low degradation rate of 0.015V*kh-1 under 0.2 A/cm2. YCC-based ceramic anodes are therefore critical for the advancement of LT-SOFC technology.


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Introduction Solid oxide fuel cells (SOFCs) are energy efficient, all-solid-state electrochemical devices that convert the chemical energy of fuels such as hydrogen or hydrocarbons into electrical energy. 1-3 Conventionally, SOFCs operate at high temperatures of ≥ 800°C, such high-temperature operation leads to various issues such as material degradation, sealing problems, electrode densification, and interdiffusion of elements.4 To overcome these problems and to extend the choice of material selections, tremendous efforts have been made to lower the operating temperature of SOFCs (≤ 600°C). Lowering the temperature without compromising SOFC performance is critical to reducing the overall system cost. A low-temperature SOFC (LT-SOFC) can improve reliability and has enormous potential to satisfy the power requirements in stationary and transportation sectors.5 Ni-based cermets have been considered the state-of-the-art anode for SOFCs because of their irreplaceable catalytic activity for hydrogen oxidation; however, Ni-based anode supports (e.g. NiYSZ) are prone to carbon formation when operated on hydrocarbon fuels and are intolerant to H2S containing fuels (e.g. natural gas)6. Further, as an anode support, they are reduction-oxidation (redox) unstable because Ni undergoes huge volumetric expansion upon successive redox process.3, 7-9 As such, alternative anodes are required to replace the standard Ni-based cermet anode.10-12 The desired characteristics of an alternative SOFC anode are high electronic conductivity, redox stability, an appropriate thermal expansion coefficient matching that of



electrolyte, catalytic activity for H2/hydrocarbon oxidation, chemical stability, and sufficient porosity in combination with mechanical strength. Mixed ionic and electronic conductors (MIEC) based perovskite oxides are the potential alternative anode materials for SOFCs.3 The main advantage of MIEC-based anodes is an extension of the reaction zones for fuel oxidation beyond the anode/electrolyte interface. 13-14 Y0.8Ca0.2CrO3-δ is a MIEC perovskite oxide that has been employed as SOFC interconnects, and recently identified as a potential anode material for SOFCs.15-18 La0.75Sr0.25Cr0.5Mn0.5O3-δ is another Cr-containing MIEC that had been explored thoroughly; however, in humidified gas conditions, its perovskite structure can be decomposed due to the precipitation of La(OH)3.19 In contrast, Y0.7Ca0.3CrO3-δ has shown better stability without forming hydroxide under SOFC operating conditions. In particular, transition metal(s) substituted Y0.7Ca0.3CrO3-δ is more stable over a wide temperature range (25-1200 °C).15 This suggests that doped-Y0.7Ca0.3CrO3-δ can meet the stringent requirements of LT-SOFC anodes. In this study, we report a newly developed Y0.7Ca0.3Cr1-xCuxO3-δ (x=0, 0.05, 0.12 and 0.20) (YCC) perovskite oxide as the redox-stable anodes for LT-SOFCs. These compositions consist of a transitional metal dopant (e.g., Cu) to improve the conductivity and shrinkage characteristics of YCC anode materials. Further, YCC provides electronic conduction for current collection and supports the infiltrated nano-electrocatalyst (10 % Ni in Gd0.1Ce0.9O2-δ (GDC) for efficient fuel oxidation reactions. The performance of the anode material at low temperatures (500-600°C) was determined using GDC-based cathode-supported SOFCs, wherein porous GDC cathode was infiltrated with La0.6Sr0.4CoO3-δ(LSC) nanoparticles. Further, the performance of the SOFC under a constant load was examined to determine the long-term stability.


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Experimental Preparation and characterization of Y0.7Ca0.3Cr1-xCuxO3-δ (x=0, 0.05, 0.12 and 0.20): A standard solid-state route was used to synthesize variants of Y0.7Ca0.3Cr1-xCuxO3-δ (x=0, 0.05, 0.12 and 0.20) anode, known as YCC (0-20) in this article. Stoichiometric amounts of yttrium oxide (Y2O3, AlfaAesar), calcium carbonate (CaCO3, Sigma-Aldrich), chromium oxide (Cr2O3, Inframet-Advanced Materials) and copper oxide (CuO, Alfa-Aesar) were ball-milled for 24 h in ethanol. The mixed oxides were then dried to evaporate the ethanol and heat-treated at 1050°C for 6 h to produce a perovskite oxide. The resulting phase was determined using a Bruker D8 powder X-ray diffractometer (XRD) with Cu Kα radiation. The unit cell parameters were obtained using Le Bail fitting procedure (TOPAS software), which was then fitted to the orthorhombic structure with a space group pmmm. To investigate the compatibility between Y0.7Ca0.3Cr0.88Cu0.12O3-δ anode and GDC electrolyte components, their respective powders were mixed in the ratio of 1:1 wt. % and was then compacted using a hydraulic press. The powder compactions were sintered at 1000, 1100 and 1200°C for 4h and the resulting pellets were analyzed using X-ray diffraction. The pellets for DC conductivity determination were prepared by sintering the compacted powder at 1350°C for 4 h in air. The densities of the sintered samples were determined using Archimedes principle (buoyancy method) with Mettler Toledo’s Density Kit. The shrinkage measurements were made by measuring the geometrical changes before and after sintering. DC electrical conductivity measurements were performed on fully sintered YCC(0-20) rectangular bars. Silver wires and silver paste were used as leads and current collectors, respectively, attached to a Keithley 2400 source meter. The measurements were made using an in-house reactor in which gas conditions are controlled. For electrical conductivity measurements in the range 400-650°C, the samples were heated up in N2 and changed to 10%H2/87%N2/3%H2O. The samples were allowed 5 ACS Paragon Plus Environment


to equilibrate in this gas condition at 650°C over 16 h and the measurements were made at 50°C intervals down to 400°C. Subsequently, 30-minute stabilization time was provided before taking a measurement at each interval. Redox cycling experiments were performed by alternating reducing (10%H2/87%N2/3%H2O) and oxidizing conditions (air) at 650, 600, 550 and 500°C. SOFC fabrication and electrochemical characterizations: A cathode-supported SOFC configuration was employed to determine the performance of YCC12 based ceramic oxide. The YCC12 composition was particularly chosen for this investigation because it exhibits higher conductivity than other compositions of interest. The configuration consisted of 20 µm thin, dense GDC sandwiched between porous GDC cathode support (400µm) and YCC12 anode (60µm). This SOFC configuration allows for a lower sintering temperature of YCC12 on GDC to 1000°C because an unwanted reaction occurs between GDC and YCC at temperatures >1100°C. The catalytic activity in the electrodes was incorporated by solution infiltration technique. For the preparation of porous GDC, (Fuel Cell Materials) scaffold, the tape-casting technique was used and the slurry was prepared by following the standard procedures. 30 µm poly(methyl methacrylate), PMMA pore-former (24 wt. % to that of GDC content) was used in the tape-casting recipe to create sufficient porosity on the cathode side of the fuel cells. For making dense GDC tape, the slurry was formulated without the PMMA pore-former and tape-cast. Approximately 30 µm thick as-prepared GDC tape was laminated with PMMA-containing GDC green tapes (3 tapes to achieve the final thickness of 400µm) using the hot press. The laminated tapes were stepwise heat-treated to burn out the PMMA pore-formers and an organic binder, followed by sintering. The maximum firing temperature was 1450°C for 4h. The half-cell that resulted consisted of 400µm thick porous GDC scaffold and ~23µm-thick dense GDC electrolytes. The dense electrolyte side of the half-cell was then doctor blade coated with YCC12 screen printing ink and 6 ACS Paragon Plus Environment

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sintered at 1000°C for 2 h. The YCC12 ink was formulated using a commercial ink making vehicle (ESL ElectroScience, type 441) along with 10 µm PMMA pore-former (23 wt. % with respect to YCC12 solid) to obtain a porous microstructure. The infiltration procedure for LSC was adopted from the research work of Samson et al. 20 1M of LSC precursor was prepared by dissolving nitrates of lanthanum, strontium, and cobalt in water with glycine as the complexing agent. Few drops of prepared solution were infiltrated into the porous GDC scaffold and placed into the vacuum chamber for 5 minutes. A heat-treatment between successive infiltrations was made at 600°C to form LSC phase. The amount of LSC loading was 21 % of the total weight of a button cell, achieved after 10 infiltration cycles. The infiltration loading was determined by measuring the initial and final weight before and after heat treatments. LSC loaded SOFC button cells were then infiltrated with NiO-GDC on the anode side. 0.75M of NiO-GDC precursor was prepared by dissolving nitrates of nickel, cerium, and gadolinium in water. The NiO-GDC precursor contains 10 wt. % of Ni relative to GDC. Infiltration procedure similar to that of LSC was followed for NiO-GDC. Between each successive infiltration step, the sample was heat-treated at 400°C for an hour to decompose the nitrate salts.21 The amount of NiOGDC loading after 3 infiltration cycles was determined to be 1.5 wt. % of the total SOFC button cell weight. I-V characteristics and power densities of the SOFCs were determined using a Solartron 1470E. The electrochemical impedance spectroscopy (EIS) response of the fuel cells was obtained using Solartron frequency response analyzer. The impedance spectra were fitted with an equivalent circuit model using the code ZsimpWin with a complex non-linear square fitting routine (CNLS).



The active area of the fuel cell was 0.31 cm2. Gold wires and silver paste were used as leads and the current collector, respectively. The flow rates of H2 and air were 100 ml/min each. SEM of the post-tested SOFCs was performed using a Hitachi SU-70 with field emission gun equipped with a Bruker XFlash silicon drift EDS detector. A field-emission gun transmission electron microscope (JEM 2100 FE-TEM) was used for the TEM analysis. Results and Discussion The effect of Cu substitution on the Y0.7Ca0.3Cr1-xO3-δ structure is shown in Figure 1. The XRD patterns of Y0.7Ca0.3Cr1-xCuxO3-δ (x=0, 0.05, 0.12 and 0.20 termed as YCC0, YCC5, YCC12, and YCC20, respectively) with an orthorhombic symmetry is shown in Figure 1a. Up to 12% Cudoping level (YCC12), no considerable phase separation occurs; however, for 20% Cu-doping YCC20, phase segregation (CaO/Ca(OH2)) was evident from the XRD data as indicated by an arrow mark. Figure 1b is the crystal structure of YCC perovskite (ABO3). Due to the larger variation of ion sizes between 12-coordinated A-site cation (Y3+, green) and 6-coordinated B-site cation (Cr3+ , blue), dopants (Ca and Cu) occupancy in the perovskite lattice site is determined by their ionic radius. The lattice parameters, unit cell volume, and crystallite size of YCC as a function of dopant concentration are extracted and summarized in Figure 1c-d. As shown in Figure 1c, the lattice parameters change with the doping level, suggesting Cu substitutes into the perovskite lattice. Cu2+ substitutes Cr3+ at the B-site of the perovskite oxide, because the ionic radius of Cu2+ (0.073nm) is relatively close to Cr3+(0.0612nm) , while Y3+(0.091nm) is larger. As shown in Figure 1d, the unit cell volume increases with the increase in Cu dopant concentration until x ≈ 0.12. This can be attributed to the slightly larger ionic radius of substituted Cu2+ in Cr3+-site. For dopant 8 ACS Paragon Plus Environment

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concentrations x > 0.12, a decrease in unit cell volume demonstrates a deviation of Vegard-type behavior, indicating the solubility limit of Cu is between 0.12

Highly Performing Chromate-Based Ceramic Anodes (Y0.7Ca0.3Cr1

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